BANCROFT 
LIBRARY 


THE  LIBRARY 

OF 

THE  UNIVERSITY 
OF  CALIFORNIA 


f  A,  Economic  Geology,  56 
Professional  Paper  No.  42  Series  \ 

(  B,  Descriptive  Geology,  65 

DEPARTMENT  OF  THE  INTERIOR 

UNITED  STATES  GEOLOGICAL  SURVEY 

>J^ 

CHARLKS  I).  \VALCOTT,  DIRECTOR 


GEOLOGY 


OF  THE 


1WOPAH  MINIM  DISTRICT,  NEVADA 


BY 


1  SFtJRR 


WASHINGTON 

GOVERNMENT     PRINTING     OFFICE 

1  it  0  5 


CONTENTS. 


Page. 

LETTER  OF  TRANSMITTAL 19 

OUTLINE  OP  PAPER 21 

INTRODUCTION 25 

Location 25 

Topography 25 

Discovery 25 

Development 26 

Treatment  of  ores 28 

Water  supply 28 

Fuel  and  power 28 

CHAPTER  I. — GENERAL  GEOLOGY 30 

Description  of  the  rock  formations 30 

Pre-Tertiary  limestone  and  granite 30 

Tertiary  lavas 31 

Andesi  tes 31 

Earlier  andesite  (hornblende-biotite-andesite) 31 

Appearance 31 

Original  composition 31 

Present  altered  condition 31 

Location 32 

Later  andesite  (biotite-augite-andesite) 33 

Appearance 33 

Composition  and  alteration  33 

Location 34 

Relation  to  earlier  andesite 35 

Distinction  from  earlier  andesite 35 

Rhyolites  and  dacites 36 

Interrelation  of  rhyolites  and  dacites 36 

Simultaneous  eruptions 36 

Heller  dacite 37 

Location 37 

Age  of  Heller  dacite 38 

Nature  of  Heller  dacite 38 

Microscopic  characters 39 

Fraction  dacite  breccia 39 

Location 39 

Thickness 39 

Conditions  of  eruption 39 

3 


4  CONTENTS. 

CHAPTER  I. — GENERAL  GEOLOGY — Continued.  Page. 
Description  of  the  rock  formations — Continued. 
Tertiary  lavas — Continued. 

Rhyolites  and  dacites— Continued. 

Fraction  dacite  breccia — Continued. 

Relative  age 40 

Microscopic  characters 40 

Tonopah  rh yolite-dacite 41 

Appearance 41 

Microscopic  characters 41 

Alteration  near  contacts 41 

Distinction  between  the  northern  and  the  southern  areas 42 

Age  and  origin 43 

Brougher  dacite 44 

Location 44 

Volcanic  necks 44 

Contact  phenomena 44 

Dikes  from  main  masses 44 

Included  basalt : 45 

Vestiges  of  cinder  cones 45 

Formation  of  the  present  Brougher  dacite 46 

Flow  structure  and  other  phenomena 46 

Faulting  due  to  Brougher  dacite  eruptions 47 

Tuff  dikes  near  contacts 47 

Mineral  composition 48 

Oddie  rhyolite 4!) 

Location 49 

Contact  phenomena  of  Oddie-Rushton  neck 49 

Contact  phenomena  of  Ararat  neck 49 

Smaller  necks 50 

Relative  age  of  Oddie  rhyolite 60 

Mineral  composition 50 

Latest  rhyolite  or  dacite 51 

Location 51 

Age  and  origin 51 

Mineral  composition 51 

Siebert  tuff  (lake  beds) 51 

lacustrine  origin 51 

Size  of  the  lake 52 

<  )rigin  of  lake  basin 52 

Thickness  of  sedinu-nti 53 

Conditions  during  deposition 53 

Explosive  eruptions  of  the  lake  period 54 

Uplift  terminating  lake  period 54 

Basaltic  eruptions 55 

Regional  tilting  accompanying  uplift 55 


CONTENTS.  5 

CHAPTER  I. — GENERAL  GEOLOGY — Continued.  Page. 
Description  of  the  rock  formations — Continued. 
Tertiary  lavas — Continued. 

Basalt 55 

Location 55 

Relations  and  composition  of  basalt  of  Siebert  Mountain 56 

Age 56 

Chemical  composition  of  lavas 56 

Transitions  in  silica  content 56 

Analyses  of  Tonopah  lavas 57 

Chemical  composition  of  the  dacite-rhyolite  series 57 

Differences  and  relations 57 

Comparison  with  Eureka  and  Washoe  dacites  and  rhyolites 58 

Retention  of  the  term  dacite 58 

Rhyolitic  nature  of  both  dacites  and  rhyolites 59 

Determination  according  to  a  quantitative  classification 59 

Varying  composition  in  different  vents 60 

Theory  of  differentiation  of   Tonopah  lavas  from  a  uniform  type.   61 

Pseudomorphs  in  rhyolite 61 

Character  of  pseudomorphs 61 

Magmatic  origin  of  pseudomorphs 62 

Hornblende  in  Tonopah  lavas 62 

Derivation  of  rhyolite  and  basalt  from  intermediate  magma 63 

Statement  of  theory 63 

Rhyolite-basult  differentiation  theory  tested  by  analyses 63 

Complementary  nature  of  dacites  and  later  andesites 64 

Statement  of  differentiation  theory 66 

Summary  of  geological  history 66 

Age  of  the  rocks  at  Tonopah 68 

Place  of  Tonopah  lavas  in  (ireat  Basin  volcanic  history 68 

Probable  Neocene  age .". . .  69 

Infusoria  in  the  Siebert  tuffs 69 

Comparison  of  Siebert  tuffs  with  Miocene  Pah-Ute  lake  deposits 70 

Conclusion 71 

Principles  of  faulting 72 

Criteria  of  faulting 72 

Siebert  tuff  boundaries 73 

Dikes  along  fault-zones 73 

Boundaries  of  lavas 74 

Erosion  fault-scarps 74 

Scarp  phenomena  west  of  Brougher  Mountain 75 

Description  of  zigzag  scarps 75 

Zigzag  scarps  explained  by  faulting 77 

Consequences  of  explanation 77 

Zigzag  fault-scarp  on  Tonopah-Sodaville  road 77 

Origin  of  zigzag  fault-scarps 78 

Origin  of  zigzag  faults 79 


6  CONTENTS. 

CHAPTER  I. — GENERAL  GEOLOGY — Continued.  Page. 
Principles  of  faulting — Continued. 

Intrusions  controlled  by  intersecting  fractures 79 

Corroboration  of  conclusions 79 

Accuracy  of  fault  mapping 80 

Faulting  due  to  volcanic  action . 80 

Application  of  principles  beyond  map 80 

Suggested  explanation  of  Great  Basin  Tertiary  deformations 81 

End  of  volcanic  epoch 82 

CHAPTER  II. — MINERAL  VEINS 83 

Veins  of  the  earlier  andesite 83 

Period  of  mineralization 83 

Nature  of  circulation  channels 83 

Veins  due  chiefly  to  replacement 84 

Portions  of  veins  due  to  cavity  filling 85 

Cross  walls  and  ore  shoots 85 

Nature  of  mineralizing  agents 85 

Primary  ores 86 

Locality 86 

Composition 86 

Minerals 86 

Quartz 86 

Adularia 86 

Sericite 87 

Carbonates 88 

Silver  sulphides 88 

Silver  chloride 88 

Chalcopy  rite 88 

Pyrite 88 

Galena 88 

Blende 88 

Gold 89 

Analyses  of  primary  sulphide  ores 89 

Summary  of  vein  minerals 90 

Oxidation 90 

Depth  of  oxidation 90 

Cap-rocks  as  protection  from  oxidation 90 

Silver  chloride  in  oxidized  zone  of  veins 91 

Analysis  of  oxidized  ore 92 

Comment  on  the  ore  analyses 92 

Formation  of  gypsum  by  oxidizing  waters 94 

Secondary  sulphides 94 

Pyrite,  argentite,  and  native  silver 94 

Argentite,  polybasite,  and  chalcopyrite  in  druses 95 

Comparison  of  secondary  sulphides  at  Neihart  and  Tonopah 95 

Evidence  favoring  secondary  deix>sition  of  sulphides  by  descending  waters 96 


CONTENTS.  7 

CHAPTER  II. — MINERAL  VEINS — Continued.  Page. 

Veins  of  the  Tonopah  rhyolite-dacite  period 96 

Characteristics  of  Tonopah  rhyolite-dacite  veins 97 

Age  of  Tonopah  rhyolite-dacite  veins 99 

General  restriction  of  veins  to  rhyolite-dacite 99 

Effect  of  waters  producing  the  Tonopah  rhyolite-dacite  veins  on  earlier-formed  veins.  100 

The  calcitic  veins  of  Ararat  Mountain 101 

The  rhyolite  of  Ararat  a  volcanic  plug 101 

Flow  brecciation  near  contact 102 

Fissure- veins  in  the  rhyolite  plug 102 

Fissures  due  to  movement  after  consolidation 104 

Paragenesis  of  vein  materials 104 

Composition  of  vein-forming  waters 104 

CHAPTER  III. — PRESENT  SUBTERRANEAN  WATER 105 

Water  encountered  in  mining  operations 105 

Outcropping  water-zones 107 

Distribution  and  explanation-  of  water  zones 107 

Usual  absorption  of  precipitation  by  rocks 107 

CHAPTER  IV. — PHYSIOGRAPHY 109 

Origin  of  the  range  of  hills 109 

Sketch  of  Tertiary  and  Pleistocene  erosion 109 

General  features 109 

Measures  for  the  amount  of  material  eroded 110 

Features  of  erosion  in  arid  climates Ill 

Precipitation  in  region  near  Tonopah 112 

Dependence  at  Tonopah  of  topographic  relief  upon  rock  resistance 113 

Effects  of  faulting  upon  the  topography 114 

CHAPTER  V. — DESCRIPTIVE  GEOLOGY  OP  MINES  AND  PROSPECTS 115 

The  known  earlier  andesite  veins 115 

Mizpah  vein  system 115 

Mizpah  vein 115 

Extent  of  vein ; 115 

Limitation  of  vein  by  Mizpah  fault 115 

Limitation  of  vein  by  Burro  fault 115 

Limitation  of  vein  by  Siebert  fault 115 

Vein  structure 117 

Effects  of  transverse  premineral  fractures 119 

Cross  walls 119 

Branching  veins 119 

Origin  of   ore  shoots 119 

Post-mineral  faults  and  fractures 122 

Vein  composition 122 

Secondary  nature  of  ore  minerals 124 

Eearrangement  of  values  during  oxidation 124 


8  CONTENTS. 

CHAPTER  V. — DESCRIPTIVE  GEOLOGY  OF  MINES  AND  PROSPECTS — Continued.  Page. 
The  known  earlier  andesite  veins — Continued. 
Mi/pah  vein  system — Continued. 

Geology  of  the  Desert  Queen  shaft 125 

Intrusive  nature  of  rhyolite  contact 125 

Variable  attitude  of  Mount  Oddie  intrusive  contact 126 

Mizpah  vein  in  Desert  Queen  workings 126 

Formations  encountered  in  the  lower  workings 127 

The  Burro  veins 127 

Vein  structure 127 

Strength  and  extent  of  the  Burro  veins 129 

Valley  View  vein  system 129 

The  Valley  View  veins  on  Mizpah  Mill 129 

Cross  veins  and  allied  phenomena 129 

Vein  structure  and  origin 130 

Ore  in  the  vein 132 

The  Valley  View  vein  system  underground 132 

Veins  in  the  Valley  View  workings 132 

The  Valley  View  fault 133 

Veins  in  the  Stone  Cabin  workings 135 

Veins  in  the  Silver  Top  workings 136 

The  Stone  Cabin-Silver  Top  veins  a  part  of  the  Valley  View  vein  group...  137 

Correlation  of  veins  in  different  mines 137 

Effect  of  the  Valley  View  fault 137 

Hypotheses  to  explain  fault  movement 137 

Amount  of  vertical  separation  of  Valley  View  fault 138 

Fraction  No.  1  vein 140 

Discovery  and  development 140 

Nature  and  relations  of  the  Fraction  veins 140 

The  northeast  (Fraction )  fault  system 141 

The  northwest  faults 144 

Cause  of  faulting 146 

Composition  of  vein 146 

Fraction  No.  2  workings 147 

Rocks  exposed  in  shaft 147 

Faulted  vein-fragment 147 

Tonopah  rhyolite-dacite 148 

Wandering  Boy  veins 149 

Relative  elevation  of  fault  blocks  containing  Valley  View  and  Wandering 

Boy  veins 149 

Relation  of  Valley  View  and  Wandering  Boy  veins 149 

Opposing  dips  of  the  veins  probably  original 150 

Change  of  dip  shown  by  comparison  of  the  Valley  View  and  the  Stone 

Cabin 150 

Wandering  Boy  and  Valley  View  conjugated  veins.. 151 


CONTENTS.  9 

CHAPTER  V. — DESCRIPTIVE  GEOLOGY  OF  MINES  AND  PROSPECTS— Continued.  '  Page. 
The  known  earlier  andesite  veins — Continued. 
Valley  View  vein  system — Continued. 
Wandering  Boy  veins — Continued. 

Outcrops  of  Wandering  Boy  veins 152 

Representation  of  outcropping  veins  underground 152 

Fault  systems  in  the  Wandering  Boy 152 

Displacement  of  the  Wandering  Boy  fault 153 

Cross  faulting  on  the  300-foot  level 154 

Effects  of  cross  faulting  ideally  considered 157 

Application  of  principles  to  Wandering  Boy  cross-faults 161 

The  vein  dip  as  a  factor  in  the  problem 162 

Correlation  of  veins  in  Fraction  and  in  Wandering  Boy 162 

Faults  not  corresponding  to  the  main  systems 163 

Relative  age  of  Fraction  and  Wandering  Boy  faults 163 

Ore  in  Wandering  Boy  veins 163 

Veins  of  Gold  Hill .' 164 

Gold  Hill  a  fault  block 164 

Nature  of  Gold  Hill  andesite 164 

Alteration  of  Gold  Hill  andesite 165 

Enumeration  of  Gold  Hill  veins 165 

Production  of  Good  Enough  vein 166 

Vein  structure 166 

Gold  Hill  shaft 166 

Tonopah  and  California  workings 167 

Section  exposed  in  workings 167 

California  fault 167 

Veins .- 167 

Montana  Tonopah  vein  system 167 

Montana  Tonopah  mine 167 

Absence  of  veins  in  the  later  andesite 167 

Vein  on  the  392-fcot  level 167 

Branch  vein  on  the  460-foot  level 169 

Connection  of  branch  vein  with  Montana  vein 170 

Brecciated  structure  in  the  Montana  vein 170 

Crustification  in  the  Montana  vein 171 

Conditions  of  formation  of  Montana  vein 172 

Faults  on  the  460-foot  level 1 72 

Veins  on  the  512-foot  level 173 

Easterly  pitch  of  ore  bodies 1 75 

Tonopah  rhyolite-dacite  in  the  Montana  Tonopah 175 

North  Star  workings 177 

Section  passed   through 177 

Veins '. 1 78 

Faulting 178 


10  CONTENTS. 

CHAPTER  V. — DESCRIPTIVE  GEOLOGY  OF  MINES  AND  PROSPECTS — Continued.  Page. 
The  known  earlier  andesite  veins — Continued. 
Montana  Tonapah  vein  system — Continued. 

Midway  workings 179 

Later  andesite  in  shaft 179 

Typical  early  andesite  in  shaft 179 

Tonopah   rhyolite-dacite  in  shaft 179 

Formation  exposed  by  drifting 179 

Veins  in  the  Midway 180 

Tonopah  Extension  mine 181 

Contact  of  earlier  and  later  andesites. .'. 181 

Veins  in  the  earlier  andesite 182 

Veins  in  the  Tonopah  rhyolite-dacite 183 

Other  exploratory  workings,  wholly  or  partly  in  early  andesite 184 

West   End  workings 184 

Outcrop  of  West  End  fault 184 

Rhyolite  intrusion  along  fault 184 

Character  of  andesite  above  220-foot  level 185 

Character  of  andesite  on  220-foot  level 186 

Correlation  of  andesites  in  West  End  and  Fraction  workings 186 

Extension  of  correlation  to  the  Wandering  Boy  and  to  Gold  Hill 186 

The  West  End  andesite  probably  earlier  andesite 187 

Contact  between  earlier  and  later  andesites 187 

Place  and  character  of  contact 187 

Nature  of  similar  contacts  elsewhere 187 

Tonopah  rhyolite-dacite 188 

Earlier  andesite  at  bottom  of  shaft 188 

MacNamara  workings 189 

Later  andesite  at  surface 189 

Character  of  andesite  on  200-foot  level 189 

Correlation  of  MacNamara  and  West  End  andesites 189 

Contact  of  earlier  and  later  andesites 190 

Tonopah  rhyolite-dacite  and  included  veins 190 

Explorations  on  veins  at  the  contacts  of  the  Oddie  rhyolite 191 

Wingfleld  tunnel 191 

Boston  Tonopah  shaft 192 

M iriam  shaft 193 

Desert  Queen  shaft 193 

Shafts  at  the  unmineralized  contact  of  the  Oddie  rhyolite 193 

Hi-lmont  shaft 193 

Rescue  shaft 194 

Explorations  on  veins  at  the  contact  of  the  Tonopah  rhyolite-dacite 194 

Mizpali  Extension  shaft 194 

I>ater  andesite  at  top  of  shaft 194 

Rhyolite  and  rhyolite-dacite  in  shaft 195 

Veins  at  contact  of  Tonopah  rhyolite-dacite 195 


CONTENTS.  11 

CHAPTER  V. — DESCRIPTIVE  GEOLOGY  OF  MINES  AND  PROSPECTS— Continued.  page. 
Explorations  on  veins  at  the  contact  of  the  Tonopah  rhyolite-da'cite — Continued. 

Mizpah  Extension  shaft — Continued. 

Correlation  of  the  rhyolitic  rocks  in  the  shaft 196 

Age  of  the  veins 197 

King  Tonopah  shaft 19" 

.  Geological  situation 19" 

Vein  materials 197 

Nature  of  rock  inclosing  vein  materials 197 

Correlation  of  veins  with  other  occurrences 198 

Belle  of  Tonopah  shaft 198 

Geological  conditions 198 

Veins 198 

Shafts  at  the  unmineralized  contact  of  the  Tonopah  rhyolite-dacite 199 

Butte  Tonopah  shaft 199 

Little  Tonopah  shaft 199 

Shafts  at  the  contact  of  the  Brougher  dacite 200 

Big  Tono  shaft 200 

Molly  shaft 200 

Shafts  wholly  or  chiefly  in  dacitic  tuffs 200 

New  York  Tonopah  shaft 200 

Fraction  Extension  shaft 201 

Geological  section 201 

Fault 202 

Tonopah  City  shaft 202 

Geological  section 202 

Indicated  displacement  of  fault  blocks 202 

Ohio  Tonopah  shaft 202 

Dacite  tuffs  in  shaft 202 

Later  andesite  in  shaft 203 

Solid  Tonopah  rhyolite-dacite 203 

Characteristics  of  the  rhyolite-dacite 204 

Mineralization ., 204 

Pittsburg  shaft 204 

Red  Kock  shaft 204 

Shafts  entirely  or  chiefly  in  later  andesite 205 

Halifax  shaft 205 

Golden  Anchor  shaft 205 

CHAPTER  VI. — ROCK  ALTERATION  CONNECTED  WITH  MINERALIZATION 207 

Alteration  of  the  earlier  andesite 207 

Alteration  of  earlier  andesite,  chiefly  to  quartz,  sericite,  and  adularia 207 

Alteration  of  hornblende  and  biotite 207 

Relations  of  pyrite  and  siderite 208 

Alteration  of  soda-lime  feldspar  to  quartz  and  sericite 208 

Alteration  of  soda-lime  feldspar  to  adularia 208 

Alteration  of  the  groundmass 209 


12  CONTENTS. 

CHAPTER  VI. — ROCK  ALTERATION  CONNECTED  WITH  MINERALIZATION — Continued.  page. 
Alteration  of  the  earlier  andesite — Continued. 

Alteration  of  earlier  andesite,  chiefly  to  quartz,  sericite,  and  r.dularia — Continued. 

Advanced  stage  of  alteration 209 

Occurrence  of  kaolin 209 

Alteration  of  earlier  andesite,  chiefly  to  calcite  and  chlorite 210 

Transitions  between  alteration  phases  of  earlier  andesite 210 

Different  alterations  the  effect  of  the  same  waters 210 

Refractoriness  of  potash  feldspars 211 

Meaning  of  adularia  and  albite  as  gangue  minerals 212 

Study  of  typical  specimens 213 

Microscopic  descriptions 213 

Earlier  andesite  from  lower  part  of  Siebert  shaft 213 

Earlier  andesite  from  Tonopah  and  California  shaft 213 

Earlier  andesite  from  Fraction  No.  2  shaft 214 

Earlier  andesite  from  near  Mizpah  Hill 214 

Earlier  andesite  from  near  Mizpah  vein 214 

Typical  earlier  andesite  from  Mizpah  Hill 215 

Earlier  andesite  from  hanging  wall  of  Mizpah  vein 215 

Ore  material  of  Mizpah  vein 216 

Analyses  of  described  types 216 

Differences  of  phases  expressed  by  diagrams 217 

Study  of  alterations  indicated  by  analyses 217 

Alteration  of  earlier  andesite  from  lower  part  of  Siebert  shaft 217 

Alteration  of  earlier  andesite  from  California  and  Tonopah  shaft 220 

Alteration  of  earlier  andesite  from  Fraction  No.  2  shaft 221 

Alteration  of  earlier  andesite  from  near  Mizpah   Hill 223 

Alteration  of  earlier  andesite  from  near  Mizpah  vein 224 

Alteration  of  typical  andesite  from  Mizpah  Hill 225 

Alteration  of  earlier  andesite  from  wall  of  Mizpah  vein 225 

Alteration  of  earlier  andesite  to  vein   material 226 

Maximum  alteration  located  along  the  vein  zones 226 

Composition  of  mineralizing  waters  in  the  vein  zones 227 

Relation  of  adularia  to  sericite  as  alteration   products 

Formation  and  occurrence  of  adularia 228 

Conditions  required  for  the  formation  of  adularia 228 

Adularia  as  a  metamorphic  mineral 229 

Adularia  in  veins 229 

Chemistry  of  the  alteration  of  soda-lime  feldspar  to  adularia 230 

Formation  and  occurrence  of  muscovite 231 

Conditions  required  for  the  formation  of  muscovite 231 

Muscovite  as  an  alteration  product 231 

Distinct  conditions  required  for  muscovite  and  for  biotite 232 

The  sericitic  variety  of  muscovite 232 

-  Fluorine  neoewary  to  the  formation  of  mica 232 

Chemistry  of  the  alteration  of  soda-lime  feldspars  to  sericite 233 


CONTENTS.  13 

CHAPTER  VI. — ROCK  ALTERATION  CONNECTED  WITH  MINERALIZATION — Continued.  Page. 
Alteration  of  the  earlier  andesite — Continued. 

Changes  in  rarer  constituents  during  alteration  of  earlier  andesite 233 

Resume  of  effects  of  mineralizing  waters 234 

Changes  in  waters  as  a  consequence  of  rock  alteration 235 

Propylitic  alteration  of  earlier  andesite 236 

Final  composition  of  mineralizing  waters 237 

Alteration  of  the  later  andesite 238 

Study  of  typical  specimens 238 

Nearly  fresh  later  andesite  from  Mizpah  Extension  shaft 238 

Nearly  fresh  later  andesite  from  Halifax  shaft 239 

Entirely  altered  later  andesite  from  North  Star  shaft 239 

Entirely  altered  later  andesite  from  Montana  Tonopah  shaft 240 

Analyses  of  described  types  of  later  andesite 241 

Differences  of  composition  expressed  by  diagrams 242 

Comparison  of  later  andesite  with  Washoe  and  Kureka  rocks 244 

Degree  of  alteration  of  freshest  Tonopah   later  andesite 244 

Principles  of  studying  alteration  of   later  andesite 245 

Alteration  of  later  andesite  from  North   Star  shaft 246 

Alteration  of  later  andesite  from   Montana  Tonopah  shaft 247 

Siderite  as  an  alteration  product -48 

Scarcity  of  epidote  as  an  alteration  product 250 

Composition  of  altering  waters 250 

Period  of  alteration  of  later  andesite 250 

Alteration  mainly  antecedent  to  faulting 250 

Relation  of  alteration   to  vein  formation 251 

Exudation  veinlets  in  later  andesite 251 

Metalliferous  veins  in  later  andesite 251 

Conclusion 251 

Alteration  of  the  Oddie  rhyolite 252 

CHAPTER  VII. — ORIGIN  OF  MINERAL  VEINS 253 

Origin  of  the  mineralizing  and  altering  waters 253 

Antithesis  between  waters  and  associated  rock 253 

Theory  of  atmospheric  origin  of  hot  springs 254 

Theory  of  magmatic  origin  of  hot  springs 254 

Characteristics  of  Nevada  hot  springs 256 

Coupling  of  hot  and  cold  springs 256 

The  Devil's  Punchbowl 257 

Amount  of  present  and  recent  hot-spring  action . 257 

Origin  of  extinct  hot  springs  at  Tonopah 258 

Connection  with  volcanic  eruptions 258 

Consequences  of  antithesis  between  rocks  and  waters 258 

Meaning  of  nature  of  metals  in  veins 258 

Nature  of  solfataric  action 260 

Minerals  deposited  around  fumaroles 261 

Conclusions  as  to  genesis  of  Tonopah  ores 261 


14  *V   «  CONTENTS. 

Page. 

CHAPTER  VIII.— INCREASE  OF  TEMPERATURE  WITH  DEPTH 263 

Method  of.  measurement 263 

Temperatures  in  the  Mizpah  Extension  and  the  Ohio  Tonopah 263 

Temperatures  in  the  Montana  Tonopah 264 

Temperatures  in  Mizpah  Hill  workings 264 

Thermal  surveys  on  the  Comstock 265 

Comparison  of  Comstock  and  Tonopah  data 265 

CHAPTER  IX. — COMPARISON  WITH  SIMILAR  ORE  DEPOSITS  ELSEWHERE 267 

Veins  of  Pachuca  and  Real  del  Monte,  in  Mexico 267 

Other  similar  mineral  districts  in  Mexico 269 

The  Comstock  lode 270 

Silver  City  and  De  Lamar  districts,  Idaho 271 

Relation  of  the  described  districts  to  Tonopah 273 

The  petrographic  province  of  the  Great  Basin 274 

Extension  of  the  Great  Basin  petrographic  province  into  Mexico 274 

Probable  still  further  extension  of  the  Great  Basin-Mexico  petrographic  province 275 

A  metallographic  province  coextensive  with  the  petrographic  province 276 

Origin  of  shoots  or  bonanzas  in  the  veins  of  this  nietallographic  province 276 

Existence  of  a  major  Pacific  Tertiary  petro-metallographic  zone 278 


ILLUSTRATIONS. 


PLATE  I.  Topographic  map  of  Tonopah  mining  district:. 

II.  A,  B,  View  from  near  eastern  corner  of  area  mapped  on  PI.  I,  looking  southwest; 
C,  D,  Tonopah  and  surroundings,  looking  west  from  Mount  Oddie;  E,  F, 
Panorama,  looking  south  from  Butler  Mountain 24 

III.  Map  of  mining  claims,  adapted  from  map  of  Booker  and  Bradford,  Tonopah 26 

IV.  Heller  Butte '..  . 38 

V.  A,  B,  Butler  Mountain;  C,  D,  View,  looking  northwest  from  a  point  between  Rushton 

Hill  and  Golden  Mountain;  E,  F,  View  north  from  Butler  Mountain 44 

VI.  A,  Brougher  Mountain  and  Tonopah,  seen  from  Mizpah  Hill;  B,  Butler  Mountain 

from  east  base,  showing  columnar  dacite  above  and  stratified  Siebert  tuffs  below...        46 
VII.  Diagram  showing  relative  displacement  of  fault  blocks  and  their  relation  to  the  dacite 

necks 48 

VIII.  Diagram  showing  relative  provinces  occupied  by  the  Brougher  dacite  and  the  rhyolite 

necks,  and  the  relation  of  the  fault  lines  to  the  former 50 

IX.  A,  Siebert  Mountain  from  the  northeast;  B,  Mount  Oddie  from  the  northwest 52 

X.  Face  of  Siebert  Mountain  from  the  southeast 54 

XI.  Geologic  map  of  Tonopah  mining  district 56 

XII.  Diagrammatic  map  showing  two  parallel  zigzag  south-facing  scarps 76 

XIII.  Fragment  of  Montana  vein,  actual  size,  showing  crustification 84 

XIV.  Map  showing  the  chief  veins  of  Ararat  Mountain  and  their  restriction  to  the  white 

rhyolite  plug 102 

XV.  A,  Recent  basaltic  cone  near  Silver  Peak;    B,  East  front  of  Quinn  Canyon  Range, 

showing  wash  apron  typical  of  the  region 112 

XVI.  Geologic  map  of  productive  portion  of  the  Tonopah  mining  district,  showing  out- 
cropping veins  and  regions  developed  by  underground  workings 114 

XVII.  Outcropping  veins  of  Mizpah  Hill 116 

XVIII.  Siebert  shaft,  Tonopah  Mining  Company 118 

XIX.  Horizontal  plan  of  veins  as  developed  in  midsummer  of  1903  on  Mizpah  Hill,  on 

the  plane  of  the  300-foot  level  of  the  Mizpah 120 

XX.  Cross  sections  showing  structure  of  Mizpah  vein .• 122 

XXI.  Horizontal    plan    of    veins  and   faults    in   Wandering  Boy  and   Fraction   300-foot 
levels,  together  with   projected   position  of    the  main  Fraction  and  Wandering 

Boy  faults  at  this  level 162 

XXII.  Horizontal  plan  of  veins  and  faults  in  the  Montana  Tonopah  512-foot  level 172 

XXIII.  A,   B,   C,   Pyrite  and   siderite   in  Tonopah  andesite;    D,    E,    F,  Adularia  in  early 

andesite  and  veins 208 

XXIV.  Diagrams  to  show  the  changes  in  composition  brought  about  by  the  alteration  of 

the  earlier  andesite 218 

15 


16  ILLUSTRATIONS. 

Page. 

FIG.    1.  Vertical  section  of    shaft    about  1,600  feet  east  of   Tonopah  and   California  shaft, 

showing  Fraction  dacite  breccia  and  interbreccia  tuffs 40 

2.  Vertical  sketch  section  of  trench  just  west  of  Brougher  Mountain,  showing  Tonopah 

glassy  rhyolite-dacite,  intrusive  into  Fraction  dacite  breccia 41 

3.  Vertical  section   of   part   of   tunnel    north  of  Brougher   Mountain   and  southeast  of 

Ohio  Tonopah  shaft 43 

4.  Vertical  sketch  section  showing  contact  of  intrusive  dacite  with  tuff,  southwest  base  of 

Butler  Mountain 44 

5.  Vertical  section  showing  contact  of  Golden  Mountain  dacite  with  Siebert  tuff  (lake 

beds)  east  of  Golden  Mountain 45 

6.  Horizontal  plan  showing  eddying  in  the  cooling  lava   of  a   volcanic  (dacite)  neck; 

plotting  of  strong  flow  structure  on  top  of  eastern  shoulder  of  Golden  Mountain.         46 

7.  Vertical  sketch  section  of  mud  dike  at  dacite  contact  at  a  point  on  the  east  side 

of  Butler  Mountain 48 

8.  Vertical   sketch   section   taken   at   a  point    on    the  east   side   of    Butler   Mountain, 

showing  mud  dike  in  lake  beds 49 

9.  Vertical  cross   section   of  southeast  side  of   Siebert   Mountain,  showing  relations  of 

Siebert  tuffs  (lake  beds),  basaltic  flow  and  agglomerates  and  Brougher  dacite 53 

10.  Ideal  cross  section  of  Tonopah  rocks 71 

11.  Cross  section  of  water  runway  (usually  dry)  of  Plate  XII 76 

12.  Map  showing  outcropping  veins  of  Tonopah 84 

13.  Rhyolitic  veins   (later    period)   in    Tonopah    rhyolite-dacite,   814-foot    level,   Desert 

Queen  shaft,  showing  irregularity  and  lack  of  persistence  (horizontal  plan) 98 

14.  Cross   section   of    outcropping   fissure  vein    in   Ararat    rhyolite   neck    near   margin, 

Reptile  claim,  north  of  Boston  Tonopah  shaft 102 

15.  Vertical  cross  section  of  outcropping  fissure  vein  20  feet  west  of  section  shown  in 

fig.   14 103 

16.  Vertical  cross  section  of  a  portion  of  Mizpah  vein  as  exposed   in   the  Oddie  shaft, 

showing  reverse  dip  near  the  surface 116 

17.  Vertical  cross  section  of  Mizpah  vein  along  Brougher  shaft  and  inclines 116 

18.  Vertical  cross  section  of  Mizpah  vein,  Oddie  and  McMann  lease,  showing  diverging 

walls 11" 

19.  Detail  sections  from  Mizpah  vein,  showing  the  effect  of  pre-minend  cross  fractures.  119 

20.  Sections  showing  the  splitting  of  the  Mizpah  vein 120 

21.  Diagram  showing  the  distribution  of  the  richer  ores  in  Mizpah  vein 121 

22.  Sketch  of   faulted  quartz  veinlets  in   andesite,  300-foot   level,  Mi/,pah,  just  south  of 

the  Valley  View  shaft 122 

23.  Horizontal  sketch   plan  of   [>ortion  of   the   Mizpah  vein  in  stopes  east  of   Lease  •">:.'. 

about  70  feet  from  surface,  showing  probable  compensating  faulting 123 

24.  Reproduction  of  drawing  of  model  showing  the  principal  post-mineral  fractures  and 

faults  olwerved  in  the  Mizpah  mine  workings 123 

25.  Horizontal  d'mgra mtic  plan  of  Mi/.pah  vein  as  exposed  in  the  Oddie  and  McMann 

lease,  20  to  30  feet  Iwlow  the  surface 124 

26.  Horizontal  plan  of  mine  workings,  showing  the  relation  of  the  vein  in  the  Desert 

Queen  workings  to  that  on  the  corresponding  level  of  the  Mizpah  mine 12ti 

l!7.  S.-.-tioiis  slmwinu  the  structure  of  the  Burro  No.  1  vein l-s 


ILLUSTRATIONS.  1 7 

Page. 
FIG.  28.  Sections  showing  the  structure  of  the  Valley  View  veins 131 

29.  Cross  sections  of  the  Valley  View  vein 133 

30.  Vertical  section  on  plane  of  Siebert  and  Valley  View  shafts 134 

31.  Cross  section  of  veins  in  Stone  Cabin  workings 135 

32.  Sketch  of  vertical  cut  on  the  east  wall  of  the  Silver  Top  120-foot  level,  3  feet  south 

of  main  vein,  showing  splitting  and  reuniting  of  a  minor  vein 136 

33.  Horizontal  plan  of  veins  in  Valley  View  and  Stone  Cabin  workings   on   the  plane 

of  the  Mizpah  200-foot  level,  to  show  the  probable  connection  between  the  chief 
veins  on  the  two  sides  of  the  Valley  View  fault 138 

34.  Plotting  of  the  strike  of  the  faults  in  the   Fraction  workings 141 

35.  Horizontal  plan  of  vein  and  faults  on  the  237-foot  level,  Fraction  No.  1  workings..       142 

36.  Horizontal  plan  showing  vein  and  faults  on  the  300-foot  level,  Fraction  No.  1  work- 

ings          142 

37.  Horizontal  plan  showing  veins  and  faults  on  the  400-foot  level,  Fraction  No.  1  work- 

ings         143 

38.  Stereogram  showing  nature  of  movement  along  the  main  northeast  faults  in  Frac- 

tion No.  1  workings 144 

39.  Cross  section  of  Fraction  No.  1   vein 145 

40.  Horizontal  plan  of  veins  and  faults  exposed  on  the  300-foot  level,  Fraction  workings, 

showing  the  relation  of  the  vein  fragment  in  the  Fraction  No.  2  to  the  vein  on 

the  corresponding  level  of  Fraction  No.  1 147 

41.  Hypothetical  vertical   cross  section  of  the  Valley  View  vein  system  before  faulting 

and  erosion 151 

42.  Plan  showing  outcropping  veins  near  the  Wandering  Boy  and  their  probable  rela- 

tion to  the  veins  encountered  underground 153 

43.  Vertical   section   on   the  Wandering   Boy  shaft,  showing  the  main  Wandering  Boy 

fault 154 

44.  Horizontal   plan   of   115-foot  level,  Wandering   Boy  workings,  showing   minor  vein 

and  Wandering  Boy  fault 155 

4o.  Vertical  section  along  east  drift,  300-foot  level,  Wandering  Boy  mine,  showing  fault- 
ing of  vein  155 

46.  Vertical   section   along  south   drift,    300-foot    level,  Wandering   Boy   mine,  showing 

faulting  of  vein 155 

47.  Vertical  section  showing  short  crosscut  to  east  near  south  end  of  south  drift,  300-foot 

level,  Wandering  Boy,  showing  faulting  of  vein 155 

48.  Horizontal   plan   of  Wandering  Boy,  300-foot  level,  showing  fragments  of  vein  and 

cross  faults,  with  the  general  trend  of  equal  displacement 156 

49.  Stereogram  showing  the  results  of  cross  faults  equally  spaced  and  of  equal  throw .       157 

50.  Diagram  showing  horizontal   plan  of  equal  and  equally  spaced  faults   belonging  to 

two  systems  intersecting  at  right  angles 158 

51.  Diagram  showing  course  of  line  of  equal  faulting  for  two  systems  of  faults  intersect- 

ing at  right  angles  and  having  uniform   displacements,  the  spacing  being  uniform 
within  each  system  but  different  for  each  system 159 

52.  Diagram  showing  the  diverse  courses  of  lines  of  equal  displacement  which   are  the 

result   of  two   systems  of  equal   faults   intersecting  at  right   angles  but  unequally 

spaced 159 

16843— No.  42—05 '-> 


18  ILLUSTRATIONS. 

Page 

FIG.  53.  Diagram  showing  the  line  of  equal  displacement  when  the  fault  systems  are  oblique 
to  each  other  instead  of  being  at  right  angles,  the  conditions  otherwise  being  like 
those  in  fig.  50 160 

54.  Diagram  showing  the  effect  of  cross  faults  when  the  faults  of  one  system  have  twice 

the  displacement  of  those  of  the  other  system 160 

55.  Diagram   showing   trend  of   zones   of  equal   displacement  with  given   directions  of 

downthrow 161 

56.  Section  of  Good  Enough  shaft,  Gold  Hill 166 

57.  Cross  section  of  Good  Enough  vein,  Gold  Hill,  as  exposed  in  openings  just  west  of 

shaft,  showing  same  characteristics  as  in  fig.  56 167 

58.  Horizontal  plan  of  faults  and  vein  on  the  392-foot  level  of  the  Montana  Tonopah..  169 

59.  Vertical  section  along  north  drift,  392-foot  level,  Montana  Tonopah 169 

60.  Horizontal   plan   showing  veins  and    faults  on    the   460-foot    level  of  the   Monana 

Tonopah 170 

61.  Sketch  showing  face  of  ore  of  the  Montana  vein  on  the  west  drift,  460-foot  level, 

Montana  Tonopah  mine 171 

62.  Horizontal   plan   to  show   relations   of  the  Mizpah  and  Montana  veins  on  the  400- 

foot  level  of  the  Mizpah 173 

63.  Vertical  cross  section  (sketched)  of  cross  wall  limiting  chief  ore  shoot  of  Montana  vein 

below,  as  displayed  on  the  512-foot  level  of  the  Montana  Tonopah 174 

64.  Vertical  cross  section   (sketched),  showing  effect   of   curving  and  branching  faults 

on   MacDonald  vein  in  stopes  above  the  615-foot  level  on  the  Montana  Tonopah.       174 

65.  Vertical  cross  section   (sketched),  showing  effect  of  curving  and  branching  faults 

on  MacDonald  vein  in  stopes  above  the  615-foot  level  on  the  Montana  Tonopah.  175 

66.  Cross  section  showing  geology  exposed  by  Montana  Tonopah  workings 176 

67.  Section  on  plane  of  Desert  Queen  and  North  Star  shafts 177 

68.  Section  showing  geology  exposed  by  Midway  workings 180 

69.  Diagrammatic  vertical  cross  section  of  Tonopah  Extension  vein 182 

70.  Map  showing  principal  earlier  andesite  veins  now  developed  underground  within  the 

main  productive  area;  shown  on  the  horizontal  plane  of  the  Mizpah  500-foot  level.       183 

71.  Vertical   section  through  MacXamara  and  Tonopah  Extension  shafts 191 

72.  Vertical    sketch   section  of  shallow   trench  just  north   of  Belmont   shaft,   showing 

contact  of  the  Oddie  rhyolite  intrusion  with  the  later  andesite 193 

73.  Diagram  to  show  changes  in  amount*  of  commoner  elements  during  stages  of  altera- 

tion of  earlier  andesite 218 

74.  Diagram  showing  relative  proportions  of  the  less  common  elements  during  the  stages 

of  alteration  of  the  earlier  andesite 234 

75.  Diagram  showing  changes  in  composition  during  alteration  of  the  later  andesite 242 

76.  Diagram  showing  changes  in  composition  during  alteration  of  the  later  andesite 243 

77.  Plotting  of  temperature  observations  in  the  Ohio  Tonopah,  Mizpah  Extension,   and 

Montana  Tonopah  mines,  showing  increase  of  temperature  with  depth 265 

78.  Vertical  cross  sections  of  ore  bodies  or  bonanzas  in  De  Lamar  district,  Idaho;  Corn- 

stock  lode,  Nevada;  and  Cristo  vein,  Pachuca,  Mexico 277 


LETTER  OF  TRANSMITTAL. 


DEPARTMENT  OF  THE  INTERIOR, 

UNITED  STATES  GEOLOGICAL  SURVEY, 

Washington,  D.  C.,  March  27,  1905. 

SIR:  I  transmit  herewith  the  manuscript  of  a  report  on  the  Geology  of  the 
Tonopah  Mining  District  of  Nevada,  by  J.  E.  Spurr,  and  recommend  its  publication 
as  a  professional  paper. 

The  geological  problem  presented  in  this  district  is  one  that  could  not  have 
been  solved  except  by  a  trained  petrographer,  since  the  igneous  rocks  that  carry 
the  vein  deposits  have  been  largely  covered  by  practically  barren  flows  of  more 
recent  eruptives;  hence  the  very  careful  and  thorough  study  of  the  district  made 
by  Mr.  Spurr  can  hardly  fail  to  be  of  great  practical  value  to  the  miner,  as 
well  as  of  scientific  interest  to  the  student  of  ore  deposits. 
Very  respectfully, 

S.  F.  EMMONS, 

Geologist  in  Charge  of  Section  of  Metalliferous  Deposits. 
Hon.  CHARLES  D.  WALCOTT, 

Director  United  States  Geological  Survey. 

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OUTLINE  OF  PAPER. 


Ore  deposit*  were  discovered  in  the  Tonopah  mining  district,  Nevada,  in  April,  1900,  by  James  L. 
Butler.  The  town  of  Tonopah  soon  had  a  population  of  several  thousand.  The  climate  is  arid 
and  the  water  supply  scanty. 

The  rocks  of  the  mining  district  are  all  of  immediate  volcanic  origin,  with  the  exception  of  a  series 
of  water-laid  tuffs,  which  represent  the  accumulations  of  fine  volcanic  detritus  in  a  Tertiary  lake. 
All  the  rocks  are  of  Tertiary  age,  probably  Miocene-Pliocene. 

The  first  eruptions  of  this  volcanic  epoch,  as  displayed  at  Tonopah,  were  andesite.  Two  andesites 
have  been  distinguished  —the  younger  or  earlier  andesite  and  the  later  andesite,  which  is  slightly 
more  basic  than  the  earlier  andesite.  Subsequently  rhyolite  and  dacite  eruptions  occurred  at  inter- 
vals for  a  long  time  and  produced  several  of  the  formations  mapped,  which  include  tuffs  and  flows. 
The  rhyolite  and  dacites  are  closely  connected  in  every  way.  In  one  of  the  latest  periods  of  eruption 
these  lavas  produced  the  volcanoes  whose  necks,  left  in  relief  by  the  erosion  of  the  surrounding  softer 
material,  now  form  the  hills  around  Tonopah. 

The  water-laid  fine  tuffs  were  deposited  in  this  rhyolite-dacite  volcanic  epoch  at  a  time  when  the 
eruptions  had  ceased  temporarily.  The  lake  basin  may  have  been  formed  by  a  sinking  of  the  crust 
consequent  upon  the  long-continued  volcanic  outpourings.  The  epoch  of  the  deposition  of  the  lake 
beds  was  closed  by  an  uplift  accompanied  by  regional  tilting.  A  little  basalt  was  then  thrown  out 
from  volcanic  vents,  and  cones  of  agglomeratic  dacitic  material  were  formed,  whose  once  liquid  necks 
are  now  represented  by  the  isolated  hills. 

The  area  occupied  by  the  dacitic  volcanic  necks  is  coextensive  with  the  region  of  observed  com- 
plicated faulting.  Study  leads  to  the  conclusion  that  this  faulting  was  initiated  chiefly  by  the  intrusion 
of  these  necks.  After  the  intrusion  and  subsequent  eruption  there  was  a  collapse,  a  sinking  of  the 
various  vents.  The  still  liquid  lava  in  sinking  dragged  down  with  it  adjacent  blocks  of  the  intruded 
rock.  ( 

The  silica  content  of  the  lavas  shows  a  fairly  regular  transition  between  the  different  types,  but 
there  is  a  marked  break  in  general  composition  between  the  andesite-basalts  on  the  one  hand  and  the 
rhyolite-dacites  on  the  other.  In  some  of  the  most  siliceous  rhyolites  there  appear  to  be  numerous 
pseudomorphs  after  hornblende,  which  consist  of  fresh  rhyolitic  groundmass  and  indicate  that  the 
hornblende  had  been  dissolved  and  replaced  by  the  magma.  In  the  dacitic  phases  of  the  rhyolite-dacite 
fresh  hornblende  is  occasionally  found.  In  the  audesites,  especially  the  earlier  phase,  hornblende  is 
abundant.  In  the  basalt  there  is  abundant  hornblende,  but  it  is  often  pseudomorphosed  by  magmatic 
action  into  aggregates  marked  by  crystals  of  iron  oxide.  It  is  concluded  that  in  both  the  highly 
siliceous  (rhyolitic)  and  in  the  least  siliceous  (basaltic)  magmas,  hornblende  was  developed  as  a  first 
crystallization,  which  was  unsuited  to  later  conditions.  A  change  of  magmatic  composition  since  the 
first  crystallization  is  inferred,  arid  the  original  magma  is  thought  to  have  been  intermediate  or 
andesitic.  This  theory  of  magmatic  segregation  is  tested  by  comparison  of  analyses,  and  bears  the 

21 


22  OUTLINE    OF   PAPER. 

test  well.  The  theory  is  reached  that  an  original  magma  of  composition  similar  to  that  of  the  earlier 
andesite  has  split  up  by  differentiation,  first  into  more  basic  andesite  (later  andesite)  and  siliceous 
dacite,  and  later,  by  a  continuation  of  the  process,  into  siliceous  rhyolite  and  basalt. 

The  structure  is  so  complicated  that  no  general  cross  sections  have  been  made.  Some  interesting 
information  on  faulting  has,  however,  been  obtained,  chiefly  from  mine  workings.  The  faults  are 
reversed  or  normal,  straight  or  curved,  perpendicular  or  flat.  Many  varieties  of  movement  are 
illustrated  by  them. 

The  most  important  mineral  veins  occur  in  the  early  andesite,  and  do  not  extend  into  the  overlying 
rocks.  These  veins  have  been  formed,  chiefly  by  replacement,  along  narrow-sheeted  zones,  and  have 
all  the  characteristics  of  true  veins.  Transverse  fractures  have  determined  the  position  of  cross  walls 
and  ore  shoots  by  limiting  and  concentrating  the  circulation.  The  mineralization  was  probably 
caused  by  hot  ascending  waters  immediately  after  the  earlier  andesite  eruption.  The  primary 
ores  have  a  gangue  of  quartz,  adularia,  and  some  sericite  and  carbonates,  and  contain  silver 
sulphides — such  as  argentite,  polybasite,  and  stephanite — silver  selenide,  gold  in  a  yet  undetermined 
form,  chalcopyrite,  pyrite,  and  some  galena  and  blende.  The  depth  of  oxidation  is  irregular.  In  the 
ore  of  the  oxidized  zone  no  important  changes  in  the  amount  of  gold  or  silver,  as  compared  with  the 
primary  ore,  has  been  proved  to  take  place.  The  ore  near  the  surface  is  not  a  truly  oxidized  ore, 
however,  but  is  an  intimate  mixture  of  original  sulphides  (and  selenides),  together  with  secondary 
sulphides,  chlorides,  and  oxides.  Secondary  sulphides  include  argentite  and  pyrargyrite. 

The  Tonopah  ore  deposits,  when  compared  with  others,  find  their  closest  resemblances  in  the 
Comstock  in  Nevada  and  in  the  Pachuca  and  other  districts  in  Mexico,  while  the  Silver  City  and  De 
Lamar  districts  in  Idaho  are  also  similar  in  many  respects.  These  deposits  all  occur  in  Tertiary  lavas, 
chiefly  andesitic.  The  writer  has  previously  described  the  Great  Basin  region  as  forming  part  of  a 
great  petrographic  province,  and  later  it  has  been  shown  that  this  province  extends  into  Mexico,  and 
may  reach  much  farther  northeast  and  southwest.  The  similarity  of  the  ore  deposits  in  the  district 
mentioned  indicatee  that  there  is  a  metallographic  province,  which  coincides  in  part  at  least  with  the 
petrographic  province. 

A  series  of  veins,  of  small  importance  commercially  within  the  Tonopah  district,  was  formed  after 
the  eruption  of  one  of  the  members  of  the  rhyolite-dacite  series — the  Tonopah  rhyolite-dacite.  These 
veins  may  be  large,  but  are  usually  low  grade  or  barren.  They  frequently  contain  a  greater  proportion 
of  gold  than  the  earlier  andesite  veins,  and  have  other  distinguishing  characteristics.  In  some  cases 
the  waters  accomplishing  this  latter  mineralization  probably  attacked  and  concentrated  the  ores  in  the 
earlier  andesite  veins. 

A  series  of  veins  of  still  less  importance  was  formed  after  the  eruption  of  one  of  the  later  members 
of  the  rhyolite-dacite  series — a  siliceous  rhyolite,  which  makes  up  some  of  the  hills  near  Tonopah. 
One  of  these,  Mount  Ararat,  a  denuded  volcanic  neck,  is  traversed  by  fissure  veins,  carrying  very 
little  values.  These  veins  are  restricted  to  the  neck,  and  the  openings  they  fill  were  evidently  formed 
by  an  upward  movement  of  the  plug  after  consolidation. 

Part  of  the  earlier  andeeite  ia  profoundly  altered,  chiefly  to  quartz,  sericite,  and  adularia.  Other 
portions  are  altered  chiefly  to  calcite  and  chlorite.  These  alteration  phases  are  transitional  into  one 
another,  and  were  evidently  caused  by  the  same  waters.  The  maximum  effect  of  these  waters  was 
the  formation  of  the  mineral  veins  along  their  circulation  channels.  Near  the  veins  they  effected  the 
quartz-sericite-adularia  alteration,  and  penetrating  farther  away  they  effected  the  calcite-chlorite 
alteration.  The  discussion  of  these  processes  is  followed  by  the  detailed  study  of  analyses  of 
typical  specimens.  The  conclusion  is  drawn  that  the  mineralizing  waters  were  charged  with  an  excess 
of  silica,  and  probably  of  potash,  together  with  silver,  gold,  antimony,  arsenic,  copper,  lead,  zinc,  and 


OUTLINE   OF   PAPER.  23 

selenium;  that  they  also  contained  carbonic  acid  and  sulphur,  with  some  chlorine  and  fluorine;  but 
that  they  were  noticeably  deficient  in  iron. 

The  alteration  by  thermal  waters  of  the  later  andesite  is  also  discussed.  By  comparison  of  analyses 
and  by  microscopic  studies  it  is  concluded  that  the  waters  which  produced  the  alteration  were  highly 
charged  with  carbonic  acid  and  sulphureted  hydrogen,  and  contained  magnesia,  iron,  and  lime. 
The  advent  of  the  waters  is  believed  to  have  followed  the  eruption  of  the  white  siliceous  rhyolite 
above  referred  to. 

The  composition  of  the  mineral  waters  in  the  two  cases  above  referred  to  does  not  seem  to 
correspond  with  that  of  the  volcanic  rocks  whose  eruption  their  advent  followed.  The  eruption  of 
andesite  was  followed  by  the  advent  of  siliceous  a»d  potassic  waters,  poor  in  iron;  the  eruption  of  the 
rhyolite  by  waters  rich  in  lime,  magnesia,  and  iron.  This  antithesis  may  have  some  bearing  on  the 
origin  of  these  waters.  There  are  two  theories  of  the  origin  of  hot  springs— atmospheric  and 
magmatic.  In  the  dry  Nevada  region  there  are  cold  springs  which  give  evidence  of  magmatic  origin, 
while  most  of  the  hot  springs  show  no  connection  with  atmospheric  precipitation.  The  meaning  of 
the  nature  of  the  metals  in  the-Tonopah  veins  is  also  discussed.  The  conclusion  is  reached  that  the 
waters  which  produced  the  veins  were  largely  given  off  from  the  congealing  lava  below. 

The  temperature  in  the  Tonopah  mines  shows  an  abnormally  rapid  increase  with  depth, 
comparable  to  that  in  the  Comstock. 

The  water  encountered  by  underground  workings  is  very  irregularly  distributed.  Some  of  the 
shafts  have  reached  a  depth  of  over  1,000  feet  without  encountering  any  general  body  of  ground 
water,  yet  along  certain  steeply  inclined  fracture  zones  water  is  found  sometimes  quite  near  the 
surface.  These  water  zones  are  widely  spaced  and  occur  only  in  brittle  rocks.  They  are  probably 
reservoirs  bottomed  by  impervious  clay  seams.  The  porous  rocks,  such  as  the  volcanic  breccias, 
absorb  the  precipitation  like  a  sponge,  and  no  water  has  yet  been  encountered  in  them. 

The  relief  of  the  range  of  hills  in  which  Tonopah  lies  is  primarily  due  to  the  volcanic 
accumulations.  These  Tertiary  volcanic  rocks  have  been  eroded  and  much  material  has  been 
transported  from  the  hills  into  the  adjoining  desert  valleys.  In  arid  climates  erosion  is  more  general 
than  in  moist  climates,  and  as  a  result  the  relief  is  determined  to  a  much  greater  degree  by  the 
relative  hardness  of  the  rocks.  This  feature  is  beautifully  illustrated  at  Tonopah.  The  complicated 
faulting  has  had  very  slight  effect  upon  the  topography. 


U.    6.    GEOLOGICAL  SURVEY 


VIEW    FROM    NEAR    EASTERN    CORNER   OF  Af 


' 

.- 


PANORAMA,    LOOKING  SOL 


PROFESSIONAL    PAPER    NO.    42       FL- 


APPED   ON    PLATE    I,    LOOKING   SOUTHWEST. 


NG   WEST   FROM    MOUNT   OC 


"ROM    BUTLER    MOUNTAIN. 


GEOLOGY  OF  THE  TONOPAH  MINING  DISTRICT, 

NEVADA. 


By  JOSIAH  EDWARD  SPURR. 


INTEODUOTIOK 

Location. — Tonopah  (see  PI.  I)  is  situated  in  Nye  County,  Nev.,  near  the 
Esmeralda  County  line.  It  lies  south  of  Belmont  and  about  60  miles  east  of 
Sodaville,  on  the  Carson  and  Colorado  Railway.  During  the  last  year  a  railroad 
has  been  constructed  to  connect  it  with  the  Carson  and  Colorado  Railway  at 
Rhodes,  a  short  distance  south  of  Sodaville. 

Topography. — Tonopah  is  situated  in  the  western  part  of  what  has  l>een  called 
the  Great  Basin  region.  In  this  region  parallel  north-south  mountain  ranges  and 
low,  irregular  hills  and  mesas,  having  also  in  general  a  north-south  alignment, 
alternate  with  broad,  flat,  or  gently  sloping  valleys.  On  account  of  the  ariditj'  of 
the  climate  the  valleys  and  low  hills  are  bare,  save  for  scattering  desert  shrubs, 
chiefly  sagebrush,  while  higher  up,  on  the  mountains,  there  is  a  more  abundant 
vegetation. 

At  Tonopah  the  topography  is  typical  of  volcanic  areas.  Numerous  isolated 
or  connected  irregular  hills — denuded  volcanic  necks — rise  from  a  rolling  plain. 
The  town  lies  about  6,000  feet  above  sea  level,  and  the  top  of  Butler  Mountain, 
the  highest  point  near  the  town,  has  an  altitude  of  7,160  feet  (PI.  II). 

Discovery. — In  April,  1900,  James  L.  Butler,  a  resident  of  Belmont,  left  that 
place,  with  a  camping  outfit  packed  on  burros,  to  travel  toward  the  mining  camp 
called  the  "Southern  Klondike""  and  to  prospect  the  neighboring  countn*.  The 
Southern  Klondike  lies  about  10  miles  south  of  the  present  Tonopah,  and  Butler's 
trail  lay  over  the  site  of  the  present  camp.  Observing  the  ledges  of  white  quartz 
cropping  on  Mizpah  Hill,  he  broke  off  specimens,  which  he  gave  to  the  assayer  at  the 
Southern  Klondike  camp  to  be  examined.  So  little  did  these  samples  indicate  the 
values  that  the  assayer  let  them  lie  a  while  in  his  shop,  and  then,  not  seeing  any 

«  A  camp  which  attracted  some  attention  at  the  time  referred  to,  but  which  is  now  practically  deserted. 

25 


26  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,   NEVADA. 

definite  prospect  of  financial  benefit  from  the  work,  threw  them  outside  into  his 
waste  pile. 

On  his  return  journey  to  Belmont,  Butler  broke  off  more  samples  from  the  same 
ledge.  In  Belmont  he  went  to  his  friend,  T.  L.  Oddie.  a  young  lawyer  and  miner, 
and  asked  him  to  have  them  assayed,  promising  him  a  share  of  the  claims  should 
they  turn  out  to  be  worth  anything.  Mr.  Oddie  sent  the  samples  to  an  assayer 
in  Austin,  offering  him  in  turn  a  share  in  any  possible  forthcoming  results  as 
compensation  for  the  work.  After  a  considerable  delay  the  Austin  assayer 
reported  values  of  from  $50  to  $600  per  ton  in  silver  and  gold.  Mr.  Butler  did 
not  act  promptly  on  this  news,  and  the  report  coming  to  the  Southern  Klondike 
camp,  a  party,  including  the  assayer  who  had  thrown  out  the  ore  and  who  had 
subsequently  fished  out  the  rejected  specimens  from  his  waste  pile  and  assayed 
them  with  surprising  results,  started  out  to  locate  the  veins.  They  wandered 
around  within  half  a  mile  of  the  locality,  but.  confused  by  the  similarity  of  the 
low  isolated  mountains,  they  could  not  find  the  veins  and  were  compelled  to 
return.  Finally,  on  August  27,  1900,  Mr.  Butler,  accompanied  by  his  wife,  drove 
out  from  Belmont,  and  together  they  located  the  ledges  in  due  form. 

Mr.  Butler  gave  T.  L.  Oddie,  W.  Brougher,  and  several  others  interests  in 
the  original  eight  claims  which  he  located,  now  the  property  of  the  Tonopah 
Mining  Company.  In  doing  the  location  work  two  tons  of  ore  were  sorted  out 
and  shipped  to  Selby's  smelting  company.  This  netted  about  $600,  and  from  that 
time  the  property  has  paid  for  its  own  development,  a  fact  of  which  the  locators, 
who  started  in  with  a  joint  capital  of  $25,  are  properly  proud. 

Development. — In  order  to  prove  the  value  of  the  property,  Mr.  Butler  gave 
leases,  the  lessee  to  pay  25  per  cent  royalty  on  the  ore  extracted.  Some  leases 
were  given  in  December,  1900,  and  over  a  hundred  more  in  the  spring  of  1901. 
Some  of  them  proved  enormously  remunerative,  and  it  is  estimated  that,  before  the 
end  of  1901,  the  lessees  extracted  ore  to  the  value  of  about  $4,000,000.  When  the 
leases  expired,  in  January,  1902,  the  result  had  been  relative^-  of  so  little  profit 
to  the  owners  that  no  more  were  given.  In  the  meantime  the  property  had  been 
sold  to  Philadelphia  capitalists  and  reorganized  as  the  Tonopah  Mining  Company. 
This  company  began  development  work,  shipping  only  enough  ore  to  pay  for  the 
expenses  of  development  and  the  installment  of  a  proper  plant  until  the  present 
season  (1904),  when  much  larger  shipments  have  been  made. 

It  is  a  fact  worthy  of  record  that  the  leases  given  by  Mr.  Butler  were  verbal, 
not  a  scrap  of  paper  being  used,  and  that  even  when  such  arrangements  proved 
relatively  unprofitable  to  the  mine,  as  above  stated,  the  agreements  were  observed 
to  the  letter  by  Mr.  Butler,  who,  on  selling  the  control  of  the  mine,  expressly 


U.  S.  GEOLOGICAL  SURVEY 


LITTLE  TONY  °  */P  4      LEUTUEN 


MAP    OF    MINING    CLAIMS,    ADAPTED    FROM 


PROFESSIONAL  PAPER   NO.  42     PL.  Ill 


OF    BOOKER    AND    BRADFORD,    TONOPAH. 


DEVELOPMENT.  27 

stipulated  for  the  fulfillment  of  all  his  promises.  A  similar  spirit,  worthy  of 
emulation  by  all  engaged  in  mini  ig  practice,  was  observed  in  other  respects. 
The  Austin  assayer,  for  example,  received  $32,000  for  the  assay  which  he  made. 
With  the  proof  that  considerable  quantities  of  high-grade  ore  existed  at 
Tonopah a  the  camp  soon  filled  up  with  the  usual  stirring,  excited  population  of  a 
new  mining  camp.  A  writer  in  the  Anuuai  Report  of  the  Director  of  the  Mint, 
on  the  Production  of  Precious  Metals  in  1901,  quaintly  remarks,  speaking  of  the 
conditions  in  1902: 

"Tonopah  supports  32  saloons,  6  faro  Barnes,  2  dance  houses,  2  weekly  news- 
papers, a  public  school,  2  daily  stage  lines,  2  churche?,  and  other  elements  of  internal 
prosperity.  It  is  a  very  orderly  community,  and  there  has  been  but  one  stage  rob- 
bery thus  far." 

In  the  center  of  the  town  the  Fraction  shaft,  starting  in  unmineralized  soft 
volcanic  rock,  sunk  down  and  encountered  some  rich  ore  at  a  depth  of  several 
hundred  feet.  This  fired  the  imaginations  of  the  prospector  and  the  promoter  with 
the  idea  that  ore  underlay  the  surface  formations  everywhere  and  was  to  be  had 
for  the  sinking.  Claims  a  long  distance  away  from  the  real  discoveries  were  in 
demand,  though  they  showed  no  surface  indications.  To  hold  these  claims,  samples 
assaying  something  in  gold  and  silver  were  diligently  sought  for,  and  in  some  cases 
it  was  only  an  obliging  or  careless  assayer  that  saved  the  day.  Companies  were 
organized,  treasury  stock  was  advertised  and  sold,  and  shafts  were  started  in  many 
different  places.  Four  out  of  five  of  the  shafts  or  tunnels  that  were  actually  begun 
were  desperately  forlorn  hopes,  to  speak  conservatively,  while  many  companies, 
especially  some  who  were  a  considerable  distance  from  the  discoveries,  may  safely 
be  classed  as  swindles.  Others  again — aid  this  included  most  of  those  near  the 
camp  proper — were  the  honest  investments  of  earnest  men  (PI.  III). 

In  the  winter  of  1902-3  rich  ores  were  discovered  in  the  ground  of  the  Montana 
Tonopah  shaft,  which  had  been  sunk  several  hundred  feet  through  the  overlying 
barren  andesite.  Later  on,  other  shafts  also  encountered  ore  at  a  considerable 
depth,  notably  the  Desert  Queen  shaft,  the  North  Star,  and  the  Tonopah  Extension. 
These,  however,  are  all  close  to  the  original  discoveries,  and  no  important  finds  have 
been  made  in  the  outlying  territory.  On  this  account,  in  the  summer  of  1903,  a 
decided  dullness  set  in.  Many  of  the  most  important  prospecting  and  exploration 
workings  were  closed  down  on  account  of  lack  of  funds  or  too  faint  encouragement, 
and  the  era  of  reckless  and  feverish  investment  and  activity  was  closed,  at  least  for 
the  time  being. 

"The  name  ia  Indian,  and  means  water  brush,  a  desert  shrub  whose  presence  points  to  moisture  in  the  soil 
beneath. 


28  GEOLOGY    OF    TONOPAH   MINING    DISTRICT,   NEVADA. 

Treatment  of  ores. — The  conditions  of  mining,  reducing,  and  transportation, 
which  will  be  of  great  importance  to  the  future  prosperity  of  the  camp,  have  not 
yet  been  finally  determined,  though  progress  has  been  made.  Several  million 
dollars'  worth  of  ore  has  been  marketed,  but  at  a  great  cost,  for  only  ore  containing 
gold  and  silver  to  the  value  of  $100  per  ton  or  more  was  profitable  up  to  the  time 
of  the  completion  of  the  railroad.  This  ore  had  to  be  hauled  60  miles  in  wagons, 
and  shipped  to  smelters  in  California  or  Utah.  Some  of  the  delay  in  definitely 
settling  upon  more  economical  ways  of  reduction  has  been  caused  by  practical 
*.»  "»  experiments  that  have  been  carried  on.  It  seems  to  have  been  finally  decided, 
however,  that  smelting  is  the  best  method,  since  any  milling  process  does  not 
recover  the  full  values.  A  railroad  lately  finished  from  Tonopah  to  Rhodes,  a 
point  south  of  Sodaville  on  the  Carson  and  Colorado  Railway,  has  made  trans- 
portation to  the  smelters  cheaper. 

Water  supply.— The  water  problem  is  an  interesting  and  vital  one  to  any 
enterprise  in  this  arid  region.  At  first  water  was  brought  into  camp  on  the  backs 
of  burros,  from  wells  in  the  valley  a  number  of  miles  to  the  east.  Subsequently 
water  was  developed  by  wells  in  the  hills  about  -i  miles  north  of  the  camp,  and 
led  in  by  pipes.  The  supphr,  however,  was  not  abundant.  Borings  in  the  bottom 
of  one  of  the  desert  vallej^s  near  b}1,  called  Rye  Patch,  have  developed  a  great  deal 
of  water.  Rather  unexpectedly,  also,  some  of  the  prospecting  shafts  in  the  camp 
have  struck  an  abundant  supply  of  water,  though  others  are  quite  dry.  Altogether, 
therefore,  it  appears  that  there  is  abundant  water  for  domestic,  mining,  and  milling 
purposes. 

fuel  and  power. — The  power  problem  is  also  important.  Coal  has  not  been 
much  used  in  Tonopah,  although  since  the  railroad  has  been  completed  the  cost  is 
not  so  great  as  former!}'.  For  domestic  purposes  wood  has  been  used.  A  variety 
of  scrubby  pine  (pine  nut,  pinyon)  grows  in  the  mountains  and  is  cut  and  hauled 
20  miles  or  more  to  Tonopah.  Of  course  this  is  expensive.  Some  of  the  hoists  of 
the  mines  have  been  run  by  steam  engines  fired  with  this  wood,  while  others  have 
used  gasoline.  The  balance  of  favor  at  present  seems  to  lie  with  the  wood-burning 
engines  in  regard  both  to  efficiency  and  cheapness.  In  the  White  Mountain  Range, 
about  00  miles  in  an  air  line  west  from  Tonopah,  are  many  mountain  streams  which 
have  a  great  fall  and  on  which  an  abundance  of  electric  power  could  be  generated. 
The  harnessing  of  this  water  power  and  the  transmission  of  the  electricity  seems 
feasible  if  it  can  be  made  profitable. 

Coal  is  found  about  40  miles  west  of  Tonopah,  in  the  north  end  of  the  Silver 
Peak  Range,  and  also  in  Tertiary  strata  in  the  mountains  farther  north.  It  is  a 
lignite,  or  at  best  a  very  light  bituminous  coal.  It  has  been  thus  far  rejected  by 


FUEL     AND    POWER.  29 

those  considering  the  power  problem  on  account  of  its  great  content  of  ash.  Not 
all  the  seams,  however,  are  of  the  same  character;  some  coal  can  be  found  which  is 
without  an  extraordinary  ash  percentage.  This  is  in  part  a  coking  coal  and  might 
be  efficient.  The  generation  of  gas  from  these  coals  and  the  use  of  this  gas  as  a 
fuel  is  also  a  possibility  which  should  be  carefully  considered.  While  undoubtedly 
the  material  is  not  high  grade,  it  is  worthy  of  being  considered  in  a  region  where 
other  sources  of  power  are  so  costly. 

Crude  petroleum,  chiefly  from  southern  California,  has  more  recently  come  into 
favor  as  a  fuel. 


CHAPTER    I. 
GENERAL    GEOLOGY. 

DESCRIPTION    OF    THE    ROCK    FORMATIONS. 

PRE-TERTIARY  LIMESTONE  AND  GRANITE. 

In  the  immediate  vicinity  of  Tonopah  the  rocks  are  all  Tertiary  volcanics  or 
tuffs.  Eight  or  9  miles  south  of  the  camp,  however,  there  is  limestone,  very  likely 
of  Cambrian  or  Silurian  age,  which  is  intruded  by  granitic  rock.  Limestones  and 
granites  occur  also  several  miles  north  of  Tonopah,  and  at  intervals  between 
Tonopah  and  Belmont.  At  Belmont  the  limestone,  which  is  intruded  by  granite, 
is  known  to  be  Silurian.  From  20  to  40  miles  west  of  Tonopah,  on  Lone 
Mountain  and  the  Silver  Peak  Range,  both  Cambrian  and  Silurian  limestones  are 
cut  into  by  granite. 

At  Tonopah  occasional  limestone  and  qnartzite  fragments  and  more  abundant 
blocks  of  granite  (often  pegmatitic  in  structure)  occur  in  the  volcanic  breccias. 
Their  position  shows  them  to  be  blocks  which  were  hurled  out  from  volcanoes. 
Thus  it  is  shown  that  at  an  uncertain  depth  below  the  present  surface  the  ascend- 
ing lavas  broke  through  rocks  of  this  character.  In  every  case  noted  these  inclu- 
sions were  in  extremely  glassy,  generally  light-yellow  volcanic  breccia  having  the 
composition  of  rhyolite-dacite."  Three  out  of  four  localities  are  also  on  the  borders 
of  areas  of  a  peculiar  dacite,  considered  probably  the  oldest  dacite  of  the  region 
(Heller  dacite),  though  whether  this  fact  has  any  further  significance  is  not  clear. 

At  the  northeast  base  of  Heller  Butte  in  this  glassy  Heller  dacite  there  are 
inclusions  of  angular  granitic  blocks,  often  several  feet  in  diameter.  At  the  west 
base  of  the  butte  another  bowlder  of  siliceous  granitic  rock  was  found  in  the 
dacite.  A  fragment  of  the  same  rock  was  found  on  the  borders  of  the  Heller 
dacite  in  the  southeast  part  of  the  area  mapped,  southwest  of  the  fork  in  the 
road  that  runs  southeastward  from  Tonopah.  A  similar  fragment  was  found  in 
glassy  rhyolite-dacite  at  the  south  base  of  Ararat  Mountain.  All  these  fragments 
were  probably  derived  from  a  single  underlying  granitic  mass. 

Fragments  of  altered  limestone  were  noted  in  dacite  breccias,  especially  in  the 
vicinity  of  the  New  York  Tonopah  shaft. 

a  These  two  rocks  are  intimately  allied  and  associated  in  the  Tonopah  district,  and  in  their  glassy  phases  are  often  not 
easily  distinguishable  one  from  another. 

30 


THE   BOCK    FORMATIONS.  31 

TERTIARY  LAVAS. 
ANDE8ITE8. 

EARLIER   AJJDESITE  (HORNBLENDE-BIOTITE-ANDESITE). 

Of  the  Tertiary  volcanics,  which  occupy  all  of  the  Tonopah  district  proper, 
andesite  appears  to  be  the  oldest.  The  writer  has  called  this  andesite  the  earlier 
andesite  to  distinguish  it  from  a  subsequently  erupted  rock  of  very  similar  composi- 
tion. In  the  camp  it  is  often  called  the  "lode  porphyry,"  since  in  it  the  most 
valuable  veins  lie. 

Appearance. — The  earlier  andesite  has  never  been  found  in  even  an  approxi- 
mately fresh  state,  but  is  decomposed  in  varying  degrees,  sometimes  only  moderately, 
often  intensely.  The  freshest  specimens  are  a  light  colored,  dense,  finely  porphy- 
ritic  rock,  with  small  glistening  feldspar  phenocrysts  showing  on  a  fresh  fracture. 
They  have  a  greenish  tinge,  due  to  the  presence  of  chlorite  and  similar  secondary 
minerals,  if  they  are  from  the  deeper  unoxidized  mine  levels,  and  a  yellow  tinge 
from  iron  oxide  if  they  come  from  nearer  the  surface.  On  further  alteration  the 
earlier  andesite  usually  has  become  lighter  colored  and  more  siliceous,  and  at  first 
glance  altogether  resembles  a  rhyolite;  by  another  process  of  alteration,  especially 
when  there  was  a  somewhat  greater  abundance  of  original  ferromagnesian  silicates, 
the  rock  has  become  green  of  various  shades. 

Original  composition. — From  microscopic  study  it  appears  that  the  original 
fresh  rock  was  a  hornblende-biotite-andesite,  of  medium  composition.  The  struc- 
ture is  tine  porphyritic,  with  relatively  sparse  phenocrysts  in  a  glassy  groundmass 
containing  many  microlitic  crystals  and  frequently  showing  original  flow  structure. 
The  phenocrysts  were  mostly  feldspar,  hornblende,  and  biotite,  occasionallv  quartz. 
Hornblende  and  biotite  were  about  equal  in  amount,  sometimes  one  predominating, 
sometimes  another,  and  frequently  one  occurring  in  a  given  rock  specimen  almost 
to  the  exclusion  of  the  other.  Pyroxene  (probably  augite)  was  apparently  rela- 
tively rare.  The  ferromagnesian  minerals  as  a  whole  were  not  abundant,  and  the 
rock  had  a  rather  siliceous  character.  The  feldspar  was  typically  andesine-oligoclase 
(as  determined  in  the  fresher  rock),  though  some  of  the  feldspars  ranged  from  ortho- 
clase  to  labradorite,  the  basic  varieties  being  more  abundant.  The  feldspar  crystals 
are  typically  small,  slim,  and  simple  (i.  e.,  not  compound).  Apatite  in  small  crystals 
is  abundant,  and  zircon  is  frequent. 

Present  altered  condition.- — In  the  ordinary  altered  condition  these  minerals  are 
often  completely  transformed.  No  actual  biotite  or  hornblende  has  been  found 
in  these  rocks,  although  several  hundred  specimens  have  been  studied  micro- 
scopically. These  minerals  are  represented  by  their  decomposition  products — 
quartz,  sericite,  pyrite,  siderite,  and  hematite,  sometimes  chlorite  and  calcite. 


32  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 

Frequently  their  former  presence  is  attested  only  by  the  greater  abundance  in 
certain  areas  of  ferritic  minerals,  which  form  a  rude  pseudomorph  after  the 
original  crystal.  Sometimes  only  the  outline  of  the  original  is  preserved,  and 
rarely  the  original  lines  of  cleavage  can  be  traced.  Often,  on  the  other  hand, 
the  outline  has  been  lost,  and  the  decomposition  products  are  bunched  together 
so  rudely  that  the  primary  mineral  can  only  be  guessed  at. 

The  feldspar  also  is  sometimes  so  completely  altered  to  a  felt  of  secondary 
minerals,  entirely  similar  to  those  resulting  from  the  decomposition  of  the  ground- 
mass,  that  its  former  existence  can  not  be  determined  without  careful  observation. 
If  viewed  by  reflected  light  the  outlines  of  the  feldspar  crystals  can  sometimes 
be  seen.  Frequently  the  secondary  minerals  within  the  area  of  the  original  feld- 
spar are  of  slightly  coarser  grain  than  those  without.  The  feldspar  has  altered 
essentially  to  quartz  and  sericite.  Alteration  of  the  feldspar  to  adularia  or  valen- 
cianite  (a  variety  of  orthoclase)  is  also  widespread  and  important.  The  decompo- 
sition products  not  infrequently  include  kaolin,  and  occasionally  calcite,  chlorite, 
and  epidote. 

The  groundmass  undergoes  the  same  decomposition  processes  as  the  porphy- 
ritic  crystals,  becoming  generally  a  felt}7  aggregate  which  is  composed  of  secondary 
quartz  and  sericite,  but  which  includes  some  pyrite,  siderite,  and  limonite,  and 
sometimes  a  little  kaolin.  By  a  rarer  process  of  alteration  chlorite  and  calcite 
are  formed. 

As  a  result  of  these  alteration  processes  the  rock  is  usually  more  or  less 
completely  altered  to  an  aggregate  which  is  composed  of  quartz  and  sericite,  and 
which  usually  includes  some  pyrite  and  siderite,  and  frequently  adularia,  kaolin, 
and  the  iron  oxides.  Chlorite  and  calcite  are  not  so  common,  but  one  or  both  of 
these  minerals  may  be  very  abundant.  They  indicate  a  process  of  decomposition 
different  from  the  ordinary.  Chlorite  may  occur  in  a  rock  without  calcite,  and 
vice  versa.  In  one  specimen  studied,  quartz  and  chlorite  were  the  chief  products 
of  decomposition;  in  another,  quartz,  sericite,  and  chlorite.  As  a  rule,  however, 
the  rocks  may  be  divided  according  to  their  processes  of  decomposition,  as 
follows: 

1.  Quartz-sericite-adularia-pyrite-siderite  rocks;   most  abundant,    and    most    closely   connected 
with  the  metalliferous  veins. 

2.  Quartz-sericite-kaolin-iron  oxides   rock;    not   infrequent;   probably  a  modification  of  No.   1. 
Usually  plainly  associated  with  some  fault  or  other  underground  water  channel. 

3.  Chlorite-calcite  rock;   not  associated  with  the  ores. 

Location. — The  earlier  andesite  outcrops  in  only  a  limited  area,  being  chiefly 
confined  to  Mizpah  Hill  and  Gold  Hill.  It  has  been  proved  to  occur  extensively, 
however,  underneath  later  lavas. 


ANDESITES.  33 

LATER  AXDESITE    ( BIOTITE-AUGITE-AXDE8ITE). 

Appearance. — The  later  andesite  is  much  like  the  earlier  andesite,  but  is 
slightly  less  siliceous.  It  is  often  found  nearly  fresh,  and  is  in  other  places 
profoundly  decomposed,  but  the  general  process  of  decomposition  is  usually 
different  from  that  of  the  earlier  andesite.  Typically  it  is  a  rock  of  medium 
dark  color,  mottled  with  crystals  of  feldspar  and  biotite,  and  sometimes  with 
pyroxene.  It  has  generally  been  more  or  less  altered  and  has  turned  dark  green. 
Near  the  surface  the  red  of  the  oxidized  iron  combines  with  these  colors  to  form 
a  characteristic  rich  purple.  In  some  places  the  rock  has  been  thoroughly  altered 
to  calotte,  chlorite,  serpentine,  quartz,  siderite,  and  pyrite,  and  other  secondary 
minerals,  and  in  other  places  has  been  so  thoroughly  leached  as  to  be  soft  and 
white. 

Composition  and  alteration, — The  porphyritic  crystals  or  phenocrysts  are 
larger  than  in  the  earlier  andesite,  and  are  also  much  more  abundant.  There  is 
usually  a  graded  crystallization,  the  crystals  varying  from  very  large  size  by 
easy  transitions  down  to  tiny  ones,  which  pass  into  the  microlitic  groundmass. 
These  crystals  consist  chiefly  of  feldspar,  biotite.  augite.  and  hornblende. 

The  feldspar  occurs  as  stout  crystals,  which  have  an  irregular  form  caused 
by  complex  twinning  or  intergrowth.  When  fresh  enough  the  species  may 
be  determined  to  be  predominantly  between  andesine  and  labradorite,  although 
there  are  more  calcic  and  more  sodic  varieties,  varying  between  oligoclase  and 
bytownite.  «  The  feldspar  is  therefore  more  calcic  than  in  the  earlier  andesite, 
where  it  is  predominatingly  oligoclase-andesine.  It  is  usually  altered  more  or 
less  completely  to  calcite,  chlorite,  and  quartz.  Any  one  or  two  of  these 
alteration  products  may  be  scant  or  absent,  and  chlorite,  kaolin,  and  zeolites 
may  be  present. 

Biotite,  which  occurs  in  good-sized  crystals,  is  usually  bleached  to  a  light- 
green  or  transparent  color,  or  is  partly  or  wholly  recrystallized  to  muscovite, 
pyrite,  calcite,  and  siderite,  and  occasionally  a  chloride  aggregate.  Triangular 
skeletons  of  rutile  (sagenite  webs)  are  included  in  the  biotite,  and  are  left  free 
by  its  decomposition.  The  siderite,  evidently  derived  from  the  breaking  up  of 
the  iron  silicate  in  the  biotite,  general!}'  occurs  intimately  throughout  the  crystal, 
along  cleavage  lines,  etc.,  while  the  pyrite  is  usually  conlined  to  the  outside  or 
the  outer  edges  of  the  crystals. 

The  augite  is  pale  green  and  is  usually  altered.  The  alteration  products 
vary  considerably,  but  are  generally  serpentine,  chlorite,  siderite,  pyrite,  calcite, 
and  quartz.  Kaolin  and  the  zeolites  also  sometimes  occur. 

The  hornblende  is  not  abundant,  and  is  almost  always  entirely  altered.     The 
decomposition    products    are    very   similar   to    those   of  the   augite,  and   include 
16843— No.  42—05- 3 


34  GEOLOGY    OK   TOJSOPAH   MINING    DISTRICT,  NEVADA. 

chlorite,  quartz,  siderite,  frequently  calcite,  and  sometimes  sericite,  kaolin,  and 
zeolites.  Small  apatite  crystals  occur,  in  part  as  inclusions  in  the  phenocrysts. 

Magnetite  and  specular  iron  occur  as  primary  minerals,  often  abundantly.  In 
several  cases  an  isotropic  cloudy  material  of  a  brilliant  green  color,  suggesting 
chromium  or  nickel,  was  observed  in  thin  sections;  and  to  this  some  of  the  rocks 
owe,  in  part  at  least,  their  peculiarly  vivid  color.  At  times  this  secondary 
substance  seemed  to  be  derived  from  the  augite,  but  in  one  section  it  was 
plainly  derived  from  the  magnetite,  for  it  formed  rims  around  the  magnetite 
crystals.  As  analysis  showed  a  trace  of  nickel,  it  is  probable  that  the  magnetite 
contains  some  nickel  oxide."  Siderite  also  occurs  as  rims  around  the  magnetite 
and  as  pseudomorphs  after  it. 

Siderite  and  pyrite  are  more  abundant  than  in  the  early  andesite.  They  are 
usually  intimately  associated,  and  their  relations  are  interesting.  Frequently  they 
seem  to  have  been  contemporaneous  in  origin,  and  to  have  formed  side  by  side 
without  inconvenience.  As  stated  above,  however,  the  siderite  is  more  intimately 
disseminated  through  the  mass  of  the  primary  ferruginous  mineral  (biotite, 
augite,  or  hornblende)  whence  it  is  derived  than  is  the  pyrite.  Occasionally  the 
pyrite  is  altered  to  siderite  along  its  margins,  but  in  many  more  cases  the  siderite 
has  unmistakably  altered  to  pyrite  along  its  borders.  A  delicate  set  of  changes 
is  thus  indicated.  The  intimate  association  of  the  siderite  with  the  primary 
minerals,  its  frequent  replacement  by  pyrite  along  the  borders,  and  the  evident 
alteration  of  the  carbonate  to  the  sulphide  show  that  in  general  a  period  of 
pyritization  succeeded  one  of  carbonization,  or,  if  both  were  contemporaneous, 
the  period  of  pyritization  was  longer. 

The  groundmass  when  fresh  is  brown  glass,  sometimes  spherulitic,  or  it  is 
microlitic  with  brown  glass  cement.  Feldspar,  pyroxene,  and  magnetite  microlites 
may  sometimes  be  recognized.  The  groundmass  alters,  like  the  phenocrysts,  to 
quartz,  chlorite,  serpentine,  siderite,  pyrite,  calcite,  sericite-like  aggregates,  and 
occasional  zeolites  and  epidote. 

In  general  the  decomposition  products  of  the  rock  are  typically  quartz,  chlorite, 
calcite,  pvrite,  and  siderite,  but  occasionally  portions  altered  chiefly  to  quartz  and 
sericite-like  aggregates*  may  be  found. 

Location. — The  later  andesite  outcrops  in  only  the  northeastern  portion  of  the 
area  mapped,  for  in  the  southwestern  portion,  as  a  result  of  relative  subsidence 
attendant  upon  faulting,  only  higher  beds  are  exposed.  It  occurs  in  depressions 

<iln  magnetite  some  of  the  ferrous  iron  is  rarely  replaced  by  nickel;  thus  a  variety  from  Pregratten,  in  the  Tyrolese 
Alps,  in  a  schistose  serpentine,  gave  1.76  per  cent  nickel  oxide  (NiO),  together  with  traces  of  the  oxides  of  manganese, 
chromium,  and  titanium. 

t  For  some  information  on  the  real  nature  of  these  sericite-like  aggregates  see  p.  240.  It  appears  probable  that 
hydrargillite  and  talc  form  a  large  part  of  these  masses. 


ANDKSITES.  35 

between  hills  of  rhyolite  and  dacite,  because  it  is  less  resistant  to  erosion  than 
these  rocks. 

Relation  to  earlier  andesite. — The  later  andesite  directly  overlies  the  earlier 
andesite,  and  though  in  many  underground  workings  and  probably  at  every  outcrop 
the  contact  is  a  fault  contact,  caused  by  movements  subsequent  to  the  eruption  of 
the  later  andesite,  yet  in  several  shafts  one  andesite  has  been  found  apparently 
lying  undisturbed  in  its  normal  position  upon  the  other.  Such  was  the  case  in  the 
Midway,  the  West  End,  and  the  Tonopah  Extension  shafts.  In  these  places  the 
contact  was  marked  by  a  band  of  decomposed  breccia,  or  even  clay,  yet  there  was 
no  good  evidence  of  faulting.  The  quartz  veins  of  the  earlier  andesite  extend  up  to 
this  contact  in  full  strength  and  then  abruptly  disappear.  Most  likely  the  earlier 
andesite  was  deeply  eroded  and  the  veins  were  exposed  before  the  later  andesite  was 
poured  out,  and  possibly  the  decomposed  clay  or  breccia  zone  represents  the  result 
of  surface  decomposition  and  disintegration  before  the  later-andesite  period. 

Distinction  from  earlier  andesite. — The  earlier  andesite  and  the  later  andesite 
are  usually  sufficiently  distinct  in  appearance  to  permit  identification  in  the  field. 
The  later  andesite  is  generally  darker;  on  account  of  the  greater  amount  of  iron 
present  it  has  the  characteristic  strong  coloration  mentioned  above.  The  earlier 
andesite  is  characteristically  finer  grained  than  the  later,  and  contains  smaller 
and  less  abundant  porphyritic  crystals.  The  porphyritic  feldspars  in  the  earlier 
andesite  are  usually  slim,  of  simple  form,  and  almost  rectangular,  while  those 
of  the  later  andesite  are  apt  to  be  stout  and  complex  as  a  result  of  twinning. 
In  the  later  andesite  crystals  of  fresh  or  bleached  biotite  can  usually  be  seen; 
in  the  earlier  andesite  they  occur  more  rarelv. 

Similar  characteristics  serve,  as  a  rule,  for  the  microscopic  determination. 
The  phenocrysts  of  ferromagnesian  silicates — augite,  biotite,  and  hornblende — and 
their  pseudomorphs  or  decomposition  products  are  usually  more  abundant  in  the 
later  andesite.  The  typical  alteration  of  the  earlier  andesite  is  to  quartz,  sericite, 
and  a  little  pyrite;  that  of  the  later  andesite  is  to  chlorite,  quartz,  calcite,  siderite, 
and  pyrite.  While  the  character  of  the  alteration  is  a  valuable  help  in  diagnosis, 
it  is  not  by  any  means  a  sure  test,  for  in  some  cases  the  processes  of  alteration 
have  been  apparently  almost  exchanged." 

On  account  of  the  similarity  in  the  original  composition  of  the  earlier  and 
later  andesites  it  is  frequently  very  difficult,  either  from  field  or  from  microscopic 
study,  to  refer  a  specimen  to  the  proper  age.  Often  this  economically  important 
question  is  decided  by  tracing  the  doubtful  phase  into  some  decided  phase  in 
the  same  rock  body. 

nit  is  probable,  however,  that  the  sericite-like  aggregates  in  the  altered  later  andesite  are  composed  largely  of 
minerals  like  hydrargillite,  talc,  kaolin,  etc.,  rather  than  of  sericite.    See  pp.  240-241. 


36  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

KHYOLITES   AND    DACITES. 

INTERRELATION   OF   RHYOLITES   AND   DACITES. 

The  rhyolites  and  dacites  at  Tonopah  are  closely  bound  together  in  every 
way — in  chemical  and  mineralogical  composition,  in  areal  distribution,  and  in 
manner  and  time  of  eruption.  In  fact,  they  can  be  best  understood  if  considered 
as  portions  of  the  same  great  magma,  split  up,  as  the  author  would  like  to  assume, 
b}7  internal  segregation  or  magmatic  differentiation.  These  lavas  constitute  tran- 
sitions between  the  two  types  (rhyolite  and  dacite)  named  above,  and  the  dacite 
itself  is  a  very  siliceous"  one,  barely  deserving  distinction  from  the  rhyolites 
were  it  not  necessary  to  emphasize  the  distinction  between  it  and  the  still  more 
highly  siliceous  rhyolite  which  forms  some  of  the  hills  of  the  region,  such  as 
Oddie  and  Ararat.  Moreover,  although  the  rocks  of  Butler,  Brougher,  Siebert, 
and  Golden  mountains  are  distinctly  of  the  dacitic  type,  and  so  fairly  classed 
together  and  distinguished  from  the  rhyolite,  yet  different  hills  (being  denuded 
volcanic  necks  and  so  representing  separate  vents)  show  different  phases.  Golden 
Mountain,  for  example,  is  made  up  of  a  lava  which,  both  in  the  field  and  under 
the  microscope,  seems  to  be  more  closely  allied  to  the  near-by  rhyolite  than  to 
the  dacite  of  the  more  distant  eminences  in  the  lower  or  southwestern  half  of 
the  mapped  area,  such  as  Brougher  Mountain.  Chemical  tests  bear  out  this 
impression  in  large  measure.  The  fine-grained  border  facies  of  this  Golden 
Mountain  intrusion,  being  glassy  with  sparser  feldspar  phenocrysts  than  the 
normal  type,  is  indistinguishable,  without  chemical  analysis,  from  similar  rhyolite. 
The  glassy  dikes  which  extend  from  the  main  mass  are  of  the  same  character. 
Many  of  the  small  dacite-rhyolite  flows,  erupted  at  an  earlier  period  than  the 
volcanic  necks,  are  similarly  tine  grained,  and  difficult  to  classify  exactly  as  dacite 
or  rhyolite  without  numerous  and  altogether  useless  chemical  tests.  It  is  prac- 
tically certain  that  many  of  these  are  transitions  between  the  two  extreme  but 
closely  related  types. 

SIMULTANEOUS   ERUPTION'S. 

The  eruption  of  dacite  and  rhyolite,  which  succeeded  that  of  the  andesite, 
extended  over  a  long  period  and  was  characterized  by  many  variations  in  the 
rhyolite  and  dacite.  The  observed  phenomena  favor  the  conclusion  that  different 
vents  were  in  a  state  of  eruption  nearly  or  quite  simultaneously,  each  one 
contributing  its  characteristic  rock,  and  that  the  notable  alternation^  of  different 
kinds  of  lava  are  due  rather  to  the  temporary  inactivity  of  some  of  the  vents 
than  to  any  real  change  in  the  character  of  the  magma  in  the  supply  basins. 

In  order  to  describe  better  the  geologic  history  and  the  economic  geology  a 
number  of  subdivisions  have  been  made  in  the  dacite-rhyolite  series. 

«  Fur  the  use  of  the  term  "dacite"  set1  pp.  58-59. 


DACITES.  37 

HELLER    DACITE. 

Location. — Heller  Butte,  a  small,  steep  manielon  near  the  town  of  Tonopah 
(PI.  IV),  is  made  up  of  a  dacite  containing  numerous  included  fragments.  At  first 
it  was  considered  to  be  of  the  same  class  and  age  as  the  larger  buttes,  such  as 
Butler  and  Brougher  mountains,  and  since  the  latter  are  denuded  volcanic  necks 
it  was  thought  to  represent  a  smaller  contemporaneous  vent.  Afterward,  however, 
it  was  recognized  that  the  marked  abundance  of  inclusions,  the  unusually  abundant 
glassy  groundmass,  and  the  fact  that  the  porphyritic  crystals  are  frequently  larger 
than  those  of  the  dacite  of  the  larger  mountains  were  characteristic  features 
of  this  particular  rock.  Later,  other  grounds  favoring  its  assignment  to  a  quite 
different  and  earlier  period  were  discovered. 

Heller  Butte  has  a  height  of  150  to  200  feet  and  a  steep  conical  form,  elliptical 
at  the  base.  Its  rock  is  vesicular  glassy  dacite,  which  contains  inclusions  of 
pumiceous  material,  frequently  of  later  andesite,  and  occasionally  of  coarse 
siliceous  granite.  The  inclusions  of  andesite  and  granite  are  sometimes  large 
angular  bowlders,  several  feet  in  diameter.  The  form  of  the  butte  seems  to  be 
governed  by  platy  structure.  It  is  steep  and  slopes  away  in  curves  on  all  sides. 
On  the  northeast  and  southeast  sides  the  lava  is  cut  off  from  the  Fraction  dacite 
breccia  and  the  Siebert  tuffs  by  faults,  along  which  are  intruded  glassy  dikes  sent 
off  from  the  Mount  Golden  mass  of  Brougher  dacite.  On  the  western  side  the  lava 
of  the  butte  seems  to  dip  under  the  nearly  horizontal  Fi'action  dacite  breccia. 

The  Tonopah  City  shaft.  800  feet  west  of  the  Heller  dacite  area  last  referred 
to,  passed  thi-ough  300  feet  of  the  partly  fragmental,  loose  Fraction  dacite  breccia 
to  solid,  glassy  dacite  of  the  Heller  type,  which  continued  for  200  feet  more  to 
the  bottom.  The  contact  in  the  shaft  could  not  be  seen  by  the  writer  on  account 
of  the  tight  lagging,  but  it  seems  most  likely  that  the  order  is  normal  and  that 
the  lower  formation  is  the  older.  Rounded  and  subangular  inclusions  of  the  later 
andesite,  having  the  appearance  of  waterworn  pebbles,  are  frequent  in  the  Heller 
dacite  of  this  shaft,  and  are  more  abundant  toward  the  bottom.  There  are  found, 
also,  smaller  rounded  quartz  pebbles,  which  are  accounted  for  on  the  hypothesis 
that  this  lava  was  a  flow,  which  ran  over  and  caught  up  pebbles  from  an  older 
surface-gravel  deposit. 

Near  the  southeastern  edge  of  the  area  mapped  are  other  outcrops  of  lava, 
which  have  the  same  peculiar  phases — the  abundant  glassy  groundmass  and  the 
numerous  inclusions  of  foreign  materials — as  the  lava  at  Heller  Butte.  Here, 
northeast  of  the  main  road  that  crosses  the  valley,  running  between  Butler  and 
Golden  mountains,  is  a  small,  smooth  mamelon,  or  symmetrical  cone,  about  20  feet 
high  and  80  feet  in  diameter,  that  resembles  in  a  general  way  Heller  Butte,  and  is 
composed  of  the  same  lava.  This  cone  has  a  concentric,  platy  structure  parallel 


38  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,   NEVADA. 

to  its  surface,  and  to  this  it  evidently  owes  its  form.  It  is  adjoined  on  the  west 
by  a  long  tongue  of  similar  lava,  like  that  surrounding  Heller  Butte,  and  the 
whole  is  surrounded  by  the  friable  Fraction  dacite  breccia. 

Farther  west,  just  on  the  west  side  of  the  road,  is  a  similar  dome-like  hill  that 
rises  out  of  a  limited  irregular  area  of  the  same  dacite  and  is  surrounded  by  the 
Fraction  dacite  breccia.  Still  farther  west,  a  short  distance,  there  is  a  projecting 
ridge  of  Heller  dacite,  capped  by  a  mamelon  5  feet  high.  The  platy  structure  slopes 
away  from  the  ridge  on  all  sides. 

These  three  similar  cones  are  aligned  in  a  northeast-southwest  direction. 

Age  of  Heller  dacite. — The  lava  of  Heller  Butte  has  been  faulted.  It  is  thus 
older  than  the  later  intrusive  Brougher  dacite,  which  makes  up  the  important  hills 
of  the  district  (see  p.  44),  and  was  erupted  before  the  general  faulting.  It  has  been 
found  in  normal  contact  only  with  the  Fraction  dacite  breccia,  which  circumstance, 
so  far  as  it  goes,  favors  an  age  either  immediately  before  or  after  this  formation. 
On  the  southeast  side  of  Heller  Butte  the  lava  of  the  butte  is  separated  from  the 
Siebert  tuffs  by  a  fault  contact.  Examination  of  what  is  practically  the  same  fault 
(the  California  fault)  a  few  hundred  feet  farther  north,  where  the  tuffs  are  brought 
into  contact  with  the  Fraction  dacite  and  the  earlier  andesite,  both  of  which  are 
known  to-be  older  than  the  tuff,  shows  that  the  tuff  block  is  downthrown,  and 
that  the  Heller  dacite  belongs  to  a  lower'  horizon  than  the  tuff.  The  fact  that 
nowhere  has  any  of  this  Heller  dacite  been  found  between  the  Fraction  dacite  breccia 
and  the  overlying  formations  would  further  restrict  the  probabilities;  and  the  fact 
that  the  dacite  of  the  butte  appears  to  dip  under  the  Fraction  dacite  breccia  near 
Heller  Butte  and  reappear  beneath  the  breccia  in  the  Tonopah  City  shaft  favors 
the  final  assignment  of  the  Heller  dacite  to  a  period  preceding  the  formation  of 
the  Fraction  dacite  breccia.  The  inclusions  of  later  andesite  in  the  Heller  dacite 
again  fixes  the  dacite  as  later  than  the  andesite,  and  the  place  of  the  Heller  dacite 
may  be  held  to  be  between  the  later  andesite  and  the  Fraction  dacite  breccia. 

Nature  of  Heller  dacite.— The  glassy  groundmass  of  the  Heller  dacite  indicates 
cooling  at  or  not  far  from  the  surface,  and  the  apparently  waterworn  pebbles 
included  in  the  dacite  in  the  Tonopah  City  shaft  suggest  that  this  portion  of  the 
lava  was  a  flow.  At  the  same  time  the  presence  of  inclusions  of  granitic  rocks 
(sometimes  in  bowlders  several  feet  in  diameter),  as  well  as  of  the  later  andesite 
near  Heller  Butte,  shows  that  the  lava  rose  directly  from  depths  below  the  granite 
and  passed  through  this  rock  and  the  already  erupted  andesites  on  its  way  up.  A 
vent  or  volcanic  neck  is  thus  suggested  and  the  topographic  forms  of  Heller  Butte 
and  the  similar  smaller  buttes  described  with  platy  structure  parallel  to  their 
surface  offer  the  same  suggestion. 

Summarizing  the  evidence  and  inferences,  it  appears  that  the  eruption  of 
the  later  andesites  was  followed  by  an  interval  of  rest  and  erosion;  and  that  the 


DACITES.  39 

beginning  of  the  dacite-rhyolite  eruptions  was  signalized  by  the  appearance  of  the 
Heller  dacite,  which  formed  numerous  small  cones  along  lines  of  weakness  and  was 
poured  forth  in  relatively  limited  quantities. 

Microscopic  character*. — Under  the  microscope  the  Heller  dacite  shows  a 
brown  glass  groundmass,  which  is  sometimes  spherulitic  and  which  contains 
numerous  porphyritic  crystals,  nearly  always  broken,  of  quartz,  feldspar,  and 
biotite.  It  resembles  the  Brougher  dacite.  Striated  and  unstriated  feldspars  are 
about  equally  represented.  The  latter  are  probably  in  large  part  orthoclase, 
while  in  one  slide  examined  striated  feldspars  proved  to  be  andesine. 


FRACTION    DACITE    BRECCIA. 


Location. — A  considerable  part  of  the  southern  half  of  the  area  mapped  is 
covered  with  a  soft  brownish  or  greenish  rock  of  volcanic  origin.  This  rock  is 
sometimes  solid,  is  occasionally  dimly  horizontally  layered  or  packed,  is  at  times 
definitely  stratified,  and  even  contains  well-bedded  tuffs.  The  material  is  dacitic, 
essentially  like  the  Heller  and  the  Brougher  dacite.  It  does  not  occur  in  the 
relatively  elevated  northwestern  half  of  the  area  mapped  but  in  the  southeastern 
half  it  spreads  far  beyond  the  map  limits  and  occupies  large  portions  of  the  low 
areas  between  the  hills. 

Thickness. — This  formation  varies  in  volume,  but  is  frequently  several  hundred 
feet  thick.  Perhaps  the  greatest  thickness  actually  demonstrated  is  at  the  New 
York  Tonopah  shaft,  which  is  745  feet  deep  and  is  entirely  in  this  formation, 
except  for  intrusive  bodies  of  the  Tonopah  rhyolite-dacite  or  included  fragments 
of  earlier  rocks. 

Conditions  of  eruption. — In  places  the  dacite  belonging  to  this  formation  is 
nonfragmental  and  of  the  nature  of  a  flow.  But  it  is  invariably  soft  and  friable. 
It  grades  into  a  common  type  where  it  is  often  difficult  to  decide  whether  or 
not  the  rocks  are  of  fragmental  character.  They  often  consist  of  broken,  close- 
packed,  medium-sized  fragments  of  more  or  less  pumiceous  dacite,  but  under  the 
microscope  show  no  signs  of  fragmental  origin.  An  explanation  of  their  origin 
that  accounts  for  their  different  features  is  that  these  rocks  were  partly  or 
entirely  volcanic  mud  flows,  in  which  the  highly  pumiceous  and  aqueous  lava 
was  mingled  with  such  an  excess  of  heated  waters  that  it  was  partly  broken  and 
ground  up  in  the  course  of  the  flowing.  Rock  of  this  nature  grades  with  no 
sharp  line  into  thick,  unstratitied  accumulations  of  brownish  or  greenish  pumice 
fragments,  which  are  of  considerable  size,  and  which  grade  into  similar  masses 
of  smaller  pieces.  In  some  parts  of  such  deposits  a  rude  stratification  or  layer- 
ing may  be  observed,  and  occasionally  there  are  thin  layers  of  well-stratified  tuff 
(fig.  1).  These  pumice  accumulations  point  to  explosive  eruptions.  In  them  are 


40 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 


found  fragments,  some  of  which  are  several  feet  in  diameter,  of  the  earlier  andesite, 
of  the  later  andesite,  and  of  andesite  and  dacite  tutf,  which  were  probably  hurled 
out  of  the  volcanoes  in  blocks  during  these  eruptions. 

We  may  therefore  reason  that  the  period  of  the  formation  of  the  Fraction  dacite 
breccia  was  one  of  considerable  volcanic  activity,  though  not  necessarily  prolonged. 

The  volcanoes  exploded  repeatedly,  producing 
showers  of  pumice  and  ash  and  rapid  subaerial 
accumulations  of  these  materials  on  and  near  the 
slopes,  while  the  flows  were  scanty  and  so  mixed 
with  water  as  to  be  often  nearly  or  quite  mud 
flows.  .  The  upper  part  of  the  formation,  as  seen 
in  the  New  York  Tonopah  shaft,  in  the  north- 
west side  of  Siebert  Mountain,  and  elsewhere,  is 
more  fragmental  than  the  lower  portion.  Some 
solid  lava  flows  appear  interstratified  with  these 
upper  fragmental  deposits,  but  they  belong 
rather  to  the  Tonopah  glassy  rbyolite-dacite  than 
to  the  Fraction  dacite  breccia. 

Relative  age. — A  number  of  data  are  of  value 
in  the  determination  of  the  relative  age  of  this 
formation.  It  is  clearly  younger  than  the  later 
andesite,  for  it  sometimes  rests  upon  this  forma- 
tion and  typically  contains  abundant  inclusions 
of  it.  On  the  other  hand,  it  is  frequently  cut 
by  dikes  of  the  Tonopah  rhyolite-dacite  (tig.  2). 
As  already  stated,  it  overlies  the  Heller  dacite 
in  the  Tonopah  City  shaft,  and  is  most  likely 
younger  than  it.  Therefore  it  is  probably  imme- 
diately between  the  Heller  dacite  and  the  Tono- 
pah rhyolite-dacite. 

Microscopic  characters. — Microscopically  the 
rock  of  the  Fraction  dacite  breccia  is  a  biotite- 
dacite,  substantially  of  the  same  composition  as 
the  Heller  dacite  and  the  Brougher  dacite.  The 
groundmass  is  brown  glass,  often  felty,  and  fre- 
quently very  vesicular.  As  porphyritic  crystals  (usually  broken)  it  contains  quartz, 
relatively  sparse  biotite,  and  feldspar,  both  striated  and  unstriated.  The  striated 
crystals  are  relatively  considerably  more  abundant  than  in  the  Tonopah  rhyolite- 
dacite.  One  determination  showed  andesine-oligoclase. 


4  feet 


FIG.  1.— Vertical  section  of  shaft  about  1,600  feet 
east  of  Tonopah  and  California  shaft,  showing 
Fraction  dacite  breccia  and  interbreccia  tuffs. 
(1)  Finely  stratified  tuff;  (2)  sandstone  com- 
posed of  angular  and  rounded  fragments  of  da- 
clteglass;  (3)  stratified  rock,  largely  made  up 
of  pumice  fragments;  (4)  soft  dacite,  broken 
and  containing  pumice  fragments,  probably 
a  mud  flow;  (n)  like  4,  but  containing  little 
pumice. 


RHYOLITES    AND    DACITES. 


41 


TOXOPAH    RHYOLITE-DACITE. 


The  Tonopah  rhyolite-dacite  occupies  a  large  part  of  the  area  mapped.  It 
occurs  in  large  unbroken  areas  in  the  northern  corner  and  in  numerous  broken 
and  separated  areas,  bounded  by  faults,  in  the  western  corner. 

Appearance. — The  rock  has  many  different  aspects  in  the  tield,  gray,  bright 
red,  black,  and  white  being  among  the  colors  represented.  Fine  brecciation  is 
frequently  observable,  while  in  many  cases  the  rock  is  glassy,  dense,  and  charac- 
terless, especially  near  the  contacts  of  the  intrusive  masses,  or  in  the  thin  sheets. 
Under  the  microscope,  however,  the  characters  are  much  more  uniform. 

Microscopic  characters. — Characteristically  sparse  and  small  phenocrysts  occur 
in  a  glassy,  sometimes  partly  microcrystalline  brown,  gray-brown,  or  yellowish 
groundmass.  The  rock  often  possesses  flow  structure,  is  rarely  pumiceous  or 
slaggy,  and  frequently  shows  autobrecciation.  Angular  fragments  of  broken  glass, 
included  in  a  cement  of  similar  glass,  and  other  phenomena  indicate  that  the  lava 
moved  while  stiffening. 


Scale 


20  feet 


FIG.  2.— Vertical  sketched  section  of  trench  just  west  of  Brougher  Mountain,  showing  Tonopah  rhyoliie-dnoite  (6),  intru- 
sive into  Fraction  dacite  breccia  (at. 

The  porphyritic  crystals  consist  of  feldspar,  biotite,  and  quartz,  which  occur 
in  the  order  named.  Small  unstriated  blunt  crystals  of  orthoclase  are  always 
predominant  among  the  feldspars,  though  striated  and  more  elongated  crystals  are 
frequent.  Optical  determination  of  these  shows  that  they  range  from  andesine  to 
albite,  andesine-oligoclase  being  the  most  frequent  phase.  Quartz  crystals  are 
abundant  in  some  phases,  in  others  rare,  and  in  many  are  wanting,  especially  in 
the  more  glassy  phases.  Fresh  biotite  crystals  are  frequently  present  though 
rarely  abundant.  They  are  usually  small  in  size. 

A  pseudomorph  of  iron  oxide  (specular  iron?)  after  hornblende  was  observed 
in  one  case,  the  original  hornblende  having  been  resorbed  by  the  dacitic  magma. 
A  single  small  crystal  of  augite  was  found  in  one  of  the  slides,  out  of  several 
hundred  examined.  Small  original  crystals  of  specular  iron  are  often  observed. 

Alteration  near  contacts. — Silicification  and  the  production  of  secondary  minerals 
is  widespread,  especially  near  contacts  where  the  rhyolite  is  intrusive  into  older 


4:2  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 

rocks,  in  which  places  the  alteration  has  been  accomplished  mainly  by  hot-spring 
action  succeeding  the  intrusion.  Secondary  quartz,  pyrite,  and  sometimes  siderite 
are  the  chief  results,  with  exceptionally  epidote  and  adularia  and  very  rarely  a 
little  calcite.  The  quartz  ma}'  form  veinlets  in  the  rock  as  viewed  under  the 
microscope,  and  these  silicifications  may  increase  in  importance  till  they  form 
large  quartz  veins.  Vesicles  lined  with  chalcedony  were  noted  in  one  instance. 

Some  of  the  Tonopah  rhyolite-dacite  presents,  when  altered  by  the  processes 
above  referred  to,  a  close  resemblance  to  certain  highly  altered  and  silicified  phases 
of  the  earlier  andesite.  Field  work,  however,  seems  to  leave  little  doubt  as  to  the 
nature  of  such  types,  as  they  can  be  traced  into  unequivocal  rhyolite-dacites. 

These  altered  and  silicified  rhyolite-dacites,  especially  those  which  contain  no 
quartz  phenociysts,  differ  from  the  similarly  altered  earlier  andesites  in  the  scarcity 
and  smallness  of  the  phenociysts,  in  the  predominating  stout,  blunt  form  of  the 
feldspars,  which  indicates  orthoclase  where  the  alteration  is  so  great  that  no 
determination  can  be  made,  and  in  the  absence  or  scarcity  of  apatite. 

Distinction  between  the  northern  and  the  southern  areas. — The  general  character 
and  relations  of  the  Tonopah  rhyolite-dacite  differ  considerably  north  and 
south  of  an  east-west  line  across  the  middle  of  the  area  mapped.  This  line, 
probably  a  fault  line  (see  PI.  X[),  runs  up  the  main  gulch  along  which  the  road 
to  town  passes  and  into  the  town.  To  the  north  the  dacite  is  always  intrusive, 
as  its  contacts  prove.  To  the  south  the  petrographic  characters  are  in  general 
the  same  as  to  the  north,  but  the  geologic  relations  are  more  complicated.  In 
many  cases  the  dacite  is  evidently  intrusive,  while  in  other  places  it  occurs  in 
sheets  that  alternate  with  pumiceous  tuffs  and  have  all  the  appearance  of  flows. 
Under  the  microscope  also  new  features  present  themselves  and  indicate  that 
manv  of  these  rocks  are  probably  fragmental.  In  thin  sections  of  such  rocks 
the  autoclastic  glassy  dacite  has  been  finely  broken  mechanically  and  the 
fragments  are  intersected  by  dense  kaolinic  matter  into  which  iron  has  infiltrated. 
The  material  seems  to  be  an  unassorted  accumulation  of  angular  fragments, 
which  resulted  from  a  shower  of  dacitic  ash  'and  lava  fragments  during  and  after 
explosive  eruptions. 

The  southern  part  of  the  area  mapped  has  in  general  been  depressed  below 
the  northern  half  by  faulting,  and  here  internal  faulting  has  been  much  more 
active  than  in  the  other  part  (see  p.  47).  This  depressed  tract  exactly  corresponds 
with  the  area  of  intermingled  Tonopah  rhyolite-dacite  dikes,  flows,  and  tuffs.  In 
the  relatively  elevated  northern  portion  of  the  area  mapped  the  surncial  formations 
have  been  largely  worn  away,  and  only  the  intrusive  portions  of  the  Tonopah 
rhyolite-dacite  are  left,  while  in  the  southern  portion  the  corresponding  flows  and 
tuffs,  as  well  as  the  feeding  dikes,  remain. 


RHYOLITES    AND    DACITE8.  43 

Age  and  origin. — There  is  a  great  deal  of  evidence  concerning  the  age  of 
the  Tonopah  rhyolite-dacite.  In  the  northern  part  of  the  area  mapped  this 
formation  is  intrusive  into  the  earlier  andesite,  and  in  many  places  into  the  later 
andesite.  In  the  southern  half  of  the  area  it  contains  numerous  inclusions  of 
later  andesites,  as  well  as  probable  earlier  andesites,  vein  quartz,  and  granitic 
fragments.  It  is  often  intrusive  into  or  overlies  the  Fraction  dacite  breccia,  which 
therefore  in  general  seems  to  be  older  (fig.  3). 

Above  the  Fraction  dacite  breccia  proper  is  a  series  of  coarse,  pumiceous  tuffs 
which  are  rudely  layered  and  rarety  well  stratified,  and  in  which  Tonopah  rhyolite- 
dacite  sheets  are  often  interbedded,  with  no  sign  of  intrusion.  This  shows  that 
the  flows  were  poured  out  intermittently  and  alternated  with  explosive  eruptions, 
which  caused  the  great  intervening  accumulation  of  pumice  and  the  3rellow  ash 
derived  from  its  disintegration.  Occasionally  also,  but  not  commonly,  thin  sheets 
of  the  same  rhyolite-dacite  are  found  in  the  lower  part  of  the  waterlaid  Siebert 
tuffs,  which  ovejlie  the  pumiceous  unassorted  tuffs  and  breccias  and  their  inter- 
calated Tonopah  rhyolite-dacite  flows. 


/-;-;-;,-,;-;•_<  ;'ih%'^v''-""^^^^ 

,-'"'  Scale 

o  5  10  20  30  feet 

FIG.  3. — Vertical  section  of  part  of  tunnel  north  oi  Brougher  Mountain  and  southeast  of  Ohio  Tonopah  shaft,  showing 
(1)  Tonopah  glassy  rhyolite-dacite  overlying  Fraction  dacite  breccia;  (2)  faulting  subsequent  to  both;  (31  contact 
dipping  down  and  in  general  faulted  down  toward  the  volcanic  neck  of  Brougher  Mountain  (mouth  of  the  tunnel 
is  about  800  feet  distant  from  the  border  of  this  neck). 

The  geologic  position  of  the  Tonopah  rhyolite-dacite  is,  then,  pretty  clearly 
fixed.  The  eruptions  of  Fraction  dacite  breccia  were  soon  followed  by  those  of  the 
Tonopah  rhyolite-dacite,  which  mingled  with  the  Fraction  dacite,  as  indicated  by 
numerous  observations  where  these  rocks  are  intimately  associated.  The  eruptions 
of  Fraction  dacite  became  subordinate  to  those  of  the  Tonopah  rhyolite-dacite 
and  were  probably  chiefly  explosive,  contributing  material  to  the  brown  pumice 
beds,  which  are  often  several  hundred  feet  thick  and  which  alternate  with  the 
Tonopah  rhyolite-dacite  flows.  At  this  period  the  Tonopah  rhyolite-dacite 
eruption  was  at  its  height,  though  for  some  time  subsequently,  after  the  formation 
of  the  Tertiary  lake  and  the  accumulation  of  the  Siebert  tuffs  therein,  scanty 
flows  were  occasionally  and  locally  emitted.  Near  Rushton  Hill,  however,  pebbles 
of  glassy  Tonopah  rhyolite-dacite  in  a  conglomerate  at  the  base  of  the  Siebert 
tuffs  indicate  that  the  older  Tonopah  rhyolite-dacite  flows  contributed  by  erosion 
their  material  to  the  upbuilding  of  the  tuffs,  at  the  same  time  that  the  last 
tardy  flows  of  the  same  lava  were  being  brought  forth. 


44 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 


BROfC.HER   DACITE. 

Location. — This  rock  forms  most  of  the  important  hills,  the  others  being 
composed  mainly  of  rhyolite  of  the  same  age  and  origin  as  the  dacite.  The  dacite 
hills  are  Butler,  Brougher,  Siebert,  and  Golden  (Pis.  V  and  VI). 

Volcanic  necks. — These  eminences  represent  the  necks  of  former  volcanoes  or 

.the  columns  of  lava  which   rose   from   the  abyssal   regions  to   the  surface.      The 

Brougher    Mountain    neck    (as    mapped)    is    roughly    circular,    though     slightly 

elongated.  Butler  Mountain  is  ellip- 
tical, Siebert  Mountain  is  irregular, 
while  Golden  Mountain  is  elongated 
and  irregular.  Butler,  Brougher, 
and  Golden  are  all  elongated  in  an 
east- west  direction. 

Contact  phenomena. — The  proof  of 
their  origin  is  found,  chiefly  in  their 
contact  phenomena.  The  contacts 
are  usually  marked  by  a  belt  of 
dacite,  which  appears  in  the  hand 
specimen  as  a  black  glass  and  which 
is  shown  by  the  microscope  to  be  a 
glassy  phase  of  the  dacite.  This 
band  is  generally  several  feet  thick, 
and  locally  as  much  as  a  hundred 
feet.  Powerful  flow  lines,  parallel 
to  the  contact,  are  usually  observed, 
and  not  infrequently  the  actual  con- 
tact with  the  intruded  rock  can  be 
seen.  The  contacts  are  typically 
vertical,  but  they  are  by  no  means 
regular.  They  frequently  dip  out 
from  the  mountains,  and  perhaps 
more  frequently  into  them,  and  are 
often  wavy  (figs.  4  and  5).  The  earlier  andesite,  the  Fraction  dacite  breccia, 
the  Tonopah  rhyolite-dacite,  the  Siebert  tuffs,  and  the  basalt  are  at  various 
places  intruded  along  the  contact  of  these  dacite  necks,  and  thus  the  age  of  the 
dacite  is  established.  The  intruded  rocks  are  usually  hardened  and  silicified 
near  the  contact,  and  contraction  cracks  in  them  are  coated  with  chalcedony. 

Dike*  from  main  masse*. — The  contacts  are  irregular  in  detailed  horizontal 
plan,  and  tongues  are  frequently  sent  out  into  the  intruded  mass.  Along  faults 


0      5      10 


Scale 
20 


40  feet 


Flo.  4. — Vertical  sketch  section  showing  contact  of  intrusive 
dacite  with  tuff,  southwest  base  of  Butler  Mountain,  a,  Da- 
cite;  6,  dacite  glass,  contact  face;  e,  tuff,  broken  at  contact. 


U.   S.   GEOLOGICAL  SURVEY 


VIEW    LOOKING    NORTHWEST    FROM    A    POI 


PROFESSIONAL    PAPER    NO.    42       PL.    V 


EN    RUSHTON    HILL   AND   GOLDEN    MOUNTAIN. 


RHYOLITE8    AND    DACITES. 


45 


these  tongues  have  sometimes  penetrated  a  considerable  distance,  and  there  the 
lava  forms  dikes,  sometimes  thinning  to  a  very  great  degree  or  showing  only  in 
occasional  outcrops  as  "'intermittent"  dikes.  The  lava  in  these  dikes  is  glassy, 
like  the  contact  phase  of  the  main  mass. 

Included  basalt.—  Inclusions  of  basalt  were  found  in  the  Brougher  dacite  in 
several  places.  In  two  places  dikes  sent  off  from  the  Golden  Mountain  neck 
along  fault  planes  contain  augite-hornblende-basalt,  like  that  in  place  on  Siebert 
Mountain.  Besides  augite  and  hornblende,  anorthite  and  labradorite-bytownite 
feldspar  were  recognized  in  these  inclusions.  At  the  north  base  of  Butler  Moun- 
tain, within  the  conspicu- 
ous hollow  there,  the  da- 
cite  is  packed  full  of  in- 
clusions of  similar  basalt. 

Vestiges  of  cinder 
canes. — At  various  points 
around  the  base  of  Butler 
Mountain,  close  to  the  in- 
trusive neck,  is  a  coarse 
volcanic  agglomerate  of  a 
kind  not  seen  at  any 
greater  distance  from  the 
mountain.  It  consists  of 
large  angular  blocks  of 
volcanic  rocks,  alternating 
with  finer  breccia  and  ash. 
On  the  south  side  of  But- 
ler Mountain  this  material 
has  a  thickness  of  about  200  feet,  and  contains  bowlders  up  to  5  feet  in  diameter. 
These  bowlders  consist  of  lava  resembling  the  Tonopah  rhyolite-dacite.  Imme- 
diately adjacent  to  the  intrusive  Brougher  dacite  contact  on  this  side  of  the 
mountain  there  was  noted  a  bowlder  of  similar  Tonopah  rhyolite-dacite  that  was 
30  feet  in  diameter  and  lay  in  the  Sietiert  tuffs,  as  if  it  had  dropped  into  them 
when  they  were  soft  mud.  On  the  north  side  of  the  mountain  similar  agglomer- 
ates were  observed,  and  here  the  blocks  were  chiefly  of  glassy  dacite.  This 
indicates  an  accumulation  of  volcanic  cinders  and  bombs,  and  their  localization 
around  the  base  of  Butler  Mountain  shows  that  this  was  the  site  of  a  cinder 
and  bomb  cone,  which  was  built  up  as  a  result  of  the  resumption  of  volcanic- 
activity  at  the  close  of  the  tuff  period  and  perhaps  following  the  slight  basaltic 
eruptions  (manifested  within  the  area  mapped  only  on  Siebert  Mountain).  On 


10  feet 


FIG.  5.— Vertical  section  showing  contact  of  the  Golden  Mountain  dacite,  glassy 
along  the  margin,  with  Siebert  tuff  (lake  beds).  Location  due  east  of  Golden 
Peak.  Dotted  outlines  indicate  a  prospecting  pit  sunk  in  the  tuff  at  the 
contact. 


46 


GEOLOGY   OF   TONOPAH    MINING   DISTRICT,   NEVADA. 


the  southwest  slope  of  Brougher  Mountain,  also,  a  coarse  agglomerate  was 
observed  in  one  place;  and  it  is  very  likely  that  this  and  other  of  the  mountains 
of  this  group  have  had  a  similar  history. 

Formation  of  the  present  Brougher  dacite. — After  these  explosions  a  column 
of  lava  rose  and  filled  the  vent.  It  is  not  likely  that  this  lava  ever  overflowed, 
for  no  traces  of  flows  have  been  found,  and  from  such  important  vents  the  lava, 
if  poured  out,  would  be  sufficient  in  quantity  not  to  have  been  wholly  swept 
awav  bv  erosion. 


Scale  of  feet 
50  100 


Fio.  6.— Horizontal  plan  showing  eddying  in  the  cooling  lava  of  a  volcanic  (dacite)  neck;  plotting  of  strong  flow*truc. 
ture  on  top  of  eastern  shoulder  of  Golden  Mountain;  attitude  of  flow  planes  nearly  vertical,  usually  dipping  70° 
to  90°  north,  sometimes  dipping  south. 

/•'low  structure  and  other  phenomena. — The  flow  structure  in  these  necks  was 
carefully  observed,  and  the  conclusion  was  reached  that  only  at  the  contact  do 
the  flow  lines  indicate  the  direction  of  the  original  flow.  Away  from  the  contact 
the  lines  follow  all  imaginable  curves  (fig.  »5).  It  is  plain  that  after  the  rapid 
cooling  of  the  glassy  lava  near  the  contacts  the  liquid  material  standing  in  the 
neck  circulated  and  eddied  extensively  before  cooling.  There  may  b«  seen  in 


U.   S.   GEOLOGICAL  SURVtY 


*!•'-*•   7'^vw  **. 
'   ;:£•*. :>^.>v:' 

S&$$ 


.4.     BROUGHER    MOUNTAIN    AND   TONOPAH.   SEEN    FROM    MIZPAH    HILL. 


/J      BUTLER    MOUNTAIN    FROM    EAST   BASE,  SHOWING   COLUMNAR   DACITE  ABOVE  AND   STRATIFIED  SIEBERT  TUFFS   BELOW. 


RHYOLITES    AND    0ACITES.  47 

these  mountains  columnar  jointing,  small  gaping  cracks  caused  by  the  stretching 
of  the  nearly  cooled  lava,  caves  formed  by  the  collapse  of  highly  vesicular  lava, 
platy  structure  or  parting  parallel  to  the  contacts,  and  other  interesting  volcanic 
phenomena. 

Faulting  due  to  £ rougher  dacite  eruptions. — The  Brougher  dacite  is  confined  to 
the  southern  half  of  the  area  mapped.  This  general  dacite  area  is  also  coextensive 
with  the  region  of  observed  complicated  faulting,  and  a  connection  between  the 
dacite  intrusion  and  the  faulting  is  suggested.  The  faulting  occurred  subsequent 
to  the  eruption  of  all  the  rocks  older  than  this  dacite,  while  the  dacite  is  unaffected 
by  it.  This  complexly  faulted  southern  half  of  the  area  is  also  downsunken  in 
comparison  with  the  little-faulted  northern  portion.  Near  the  dacite  necks  the 
observed  faults  are  rather  more  numerous  than  elsewhere,  and  in  many  instances 
the  blocks  adjacent  to  the  dacite  have  been  downsunken  in  reference  to  blocks 
farther  awav  (PI.  VII).  From  these  intrusive  necks  the  faults  run  in  a  roughly 
radiating  fashion  and  seem  to  follow  no  regular  system  of  trend  (PI.  VIII). 
Detailed  study  of  the  contact  phenomena  of  the  dacite  shows  that  the  minute 
faults  in  the  tuffs  at  these  points  generally  have  their  downthrown  side  next  the 
dacite. 

From  these  facts  the  following  conclusions  have  been  reached.  The  faulting 
was  chieflv  initiated  by  the  intrusion  of  the  massive  dacite  necks  (the  rhyolite 
necks  were  probably  not  so  bulky)."  After  this  intrusion  and  subsequent  eruption 
there  was  a  collapse  and  a  sinking  at  the  vents.  As  the  still  liquid  lava  sank  it 
dragged  downward  the  adjacent  blocks  of  the  intruded  rock,  accentuating  the 
faults  and  causing  the  described  phenomena  of  downfaulting  in  the  vicinity  of  the 
dacite. 

In  reference  to  this  phenomenon  of  subsidence  around  volcanic  vents  Scrope* 
wrote: 

"It  would  appear,  however,  that  in  some  cases  the  eruption  of  volcanic  matter 
is  accompanied  by  the  subsidence  not  only  of  the  column  of  lava  which  had  risen 
within  the  vent,  but  also  of  the  neighboring  surface  rocks  themselves.  Several  of 
the  cinder  cones  of  New  Zealand,  as  described  by  Mr.  Heaphy,  have  been  thrown 
up  on  a  line  of  fault  in  the  Tertiary  strata  whose  upcast  forms  the  sea  cliff,  and 
show  a  clear  synclinal  depression  of  the  elsewhere  horizontal  beds,  on  either  side 
toward  the  eruptive  vent.'' 

Tuff  dikes  near  contacts. — At  some  points  along  the  contact  of  the  Butler 
Mountain  neck  with  the  Siebert  tuffs,  particularly  on  the  south  and  east  sides,  sand 
and  tuff  dikes  are  observed.  They  are  composed  of  yellow  tuff  and  included  frag- 

a  In  the  North  Star  and  Desert  Queen  mine  workings,  along  the  southeastern  part  of  Mount  Oddie,  for  example,  the 
dip  of  the  lower  contact  of  the  rhyolite  into  the  mountain  is  very  flat. 
>>  Volcanoes,  p.  225. 


48 


GEOLOGY    OF   TONOPAH   MINING    DISTRICT,   NEVADA. 


ments  of  the  glassy  contact  phase  of  the  dacite,  and  are  intrusive  into  the  Siebert 
tuffs.  Sometimes  these  dikes  are  composed  mainly  of  glassy  dacite  fragments, 
sometimes  of  clear  sand.  They  often  follow  the  exact  intrusive  contact  and  are 
never  far  from  it  (tig.  7).  Detailed  study  shows  that  these  breccias  are  truly  dikes 
that  have  been  injected  in  a  plastic  condition  (fig.  8).  This  injection  followed  the 
intrusion,  and  the  intrusive  material  was  probably  a  mixture  of  ascending  hot 
waters,  consequent  upon  the  eruption,  with  tuff  and  dacite  fragments. 

Mineral  composition. — Microscopical \\  the  Brougher  dacite   shows  a  brown, 
glassy  groundmass,  which  is  sometimes  finely  crystalline  and  contains  frequently 


Scale 


so  feet 


FIG.  7.— Vertical  sketch  section  of  dacite  contact  at  a  point  on  the  east  side  of  Butler  Mountain;  a,  gray  dacite;  b,  glassy 
dacite,  autociastic;  c,  dike  of  friable,  partly  consolidated  detritul  sand  and  angular  fragments  composed  of  material 
derived  from  the  tufl  and  from  dacite  glass;  d,  finely  stratified  Siebert  tuffs  (lake  beds);  e,  coarser  layer  of  tuffs; /,  faults. 
The  sketch  shows  clastic  dikes  consequent  upon  marginal  fissuring  around  the  dacite,  and  dowufaulting  toward  the 
contact. 

broken  porphvritic  crystals  of  quartz,  orthoclase,  andesine  or  andesine-oligoclase, 
biotite,  and  occasionally  hornblende  and  augite.  Magnetite  and  specular  iron  occur. 

In  the  field  the  Golden  Mountain  dacite  was  judged  to  be  more  siliceous  than 
that  of  the  other  mountains,  and  this  observation  has  been  borne  out  by  microscopic 
and  chemical  analysis.  It  shows,  indeed,  a  close  relation  to  the  Oddie  rhyolite. 
However,  the  Golden  Mountain  rock  is  distinguished  as  dacitic  by  the  greater 
abundance  of  porphvritic  crystals,  the  frequent  presence  of  elongated  plagioclase 
feldspars,  the  greater  amount  of  biotite,  the  characteristic  brown,  glassy  ground- 

H,  and  the  occurrence  of  occasional  augite. 


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RHYOLITES    AND    DACITES. 


ODDIE    RHYOLITE. 


Location. — A  white  siliceous  rhyolite  makes  up  Mount  Oddie  (PI.  IX,  B)  and 
Rushton  Hill,  and  extends  irregularly  in  .spurs  and  lobes  away  from  their  bases. 
A  similar  rhyolite  occurs  on  the  summit  in  an  irregular  area  at  the  northwest 
base,  but  not  on  the  slopes  of  Ararat  Mountain,  and  in  small  patches  around  the 
north  base  of  Brougher  Mountain. 

Contact  phenomena  of  Oddie- Rushton  neck. — By  the  same  method  of  reasoning 
applied  to  the  Brougher  dacite  necks,  the  conclusion  is  reached  that  Mount  Oddie 
and  Rushton  Hill  are  also  the  necks  of  ancient  volcanoes.  On  Mount  Oddie  and 
Rushton  Hill  the  rhyolite  is  intrusive.  At  man}-  points  along  the  contact  there 
is  a  vertical  flow  structure  in  the  rhyolite  and  a  platy  structure  parallel  to  it. 
The  rhyolite  of  Rushton  Hill, 
at  its  contact  with  the  later 
andesite  near  the  Rescue 
shaft,  dips  at  an  angle  of  45 : 
to  60C  away  from  the  hill. 
The  Rescue  shaft  passed  into 
this  rhyolite  and  has  con- 
tinued in  it  several  hundred 
feet  up  to  the  time  of  latest 
information. 

Near  the  contact  the 
rhyolite  is  frequently  gl;i>s\- 
and  resembles  the  Tonopah 
rhyolite-dacite;  it  has  also 
been  silicitied  in  many  places 
subsequent  to  its  eruption. 

The  rhyolite  of  Oddie 
and  Rushton  hills  sends  out 
irregular  lobes  into  the  surrounding  rocks,  which  by  reason  of  their  superior 
hardness,  as  compared  with  the  intruded  later  andesite,  form  ridges.  These  also 
are  characterized  by  vertical  flow  lines  and  platy  structure.  As  a  whole  the 
intrusion  is  elongated  in  an  east-west  direction,  parallel  to  the  previously  noted 
elongation  of  the  dacite  necks. 

Contact  phi'wmiena  of  Ararat  neck. — The  slopes  of  Ararat  Mountain  are 
formed  by  the  Tonopah  glassy  rhyolite-dacite,  which  has  already  been  described. 
The  top,  however,  is  of  rhyolite  like  that  of  Mount  Oddie,  and  the  contact 
between  the  two  is  sharp.  The  white  rhyolite  at  the  top  of  the  mountain  has 
a  roughly  circular  outline.  At  its  margin  it  is  brecciated,  sometimes  profoundly, 
16843— No.  42—05 4 


Scale 
to 


20  -feet 


FIG.  8.— Vertical  sketch  section  taken  at  a  point  on  the  east  side  of  Butler 
Mountain,  100  feet  below  the  contact  of  Butler  dacite  and  tuff,  showing 
dike  of  light-brown,  semi-consolidated  sand,  of  volcanic  origin,  containing 
angular  fragments  of  dacite  glass,  intrusive  into  Siebert  tuffs  (lake  beds). 


50  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 

and  contains  large  veins,  filled  chiefly  with  calcite,  which  do  not  extend  into  the 
Tonopah  rhyolite-dacite.  That  this  brecciated  and  veined  white  rhyolite  is 
intrusive  is  shown  by  the  fact  that  it  includes  large  blocks  of  the  later  andesite, 
where  it  comes  in  contact  with  that  rock  on  the  southwest  side  of  the  mountain. 
The  brecciation  of  the  white  rhyolite  near  its  contact  with  the  Tonopah  rhyolite- 
dacite  is  of  a  nature  between  a  flow  breccia  and  a  friction  breccia.  It  indicates 
clearly  that  movement  continued  in  this  uprising  column  of  lava  after  hardening 
and  stiffening  had  begun,  so  that  the  cooled  portions  were  broken  and  dragged 
onward  in  a  jumbled  mass  by  the  still  viscous  upward-flowing  lava.  The  upward 
strain,  continued  after  further  hardening,  resulted  in  marked  sheeting,  and  even 
in  gaping  fissures,  which  were  filled  with  calcite  and  other  minerals  by  the 
waters  which  circulated  through  them  after  the  eruption. 

Smaller  necks. — The  small  areas  of  this  rhyolite  near  the  base  of  Brougher 
Mountain  are  also  probably  necks.  They  are  circular  or  roughly  elliptical  and 
of  relatively  small  size.  One  just  northeast  of  Brougher  Mountain  is  about  400 
feet  by  150  feet  in  dimensions,  and  a  shaft  has  been  sunk  200  feet  in  it  without 
encountering  any  change  in  the  character  of  rock. 

Relative  age  of  Oddie  rhyolite. — The  later  rhyolite  is  intrusive  into  the  later 
andesite  at  many  points — into  the  Fraction  dacite  breccia  near  Brougher  Mountain, 
into  the  Tonopah  glassy  rhyolite-dacite  breccia  at  Ararat  Mountain  and  Brougher 
Mountain,  and  into  the  Siebert  tuffs  on  the  east  side  of  Rushton  Hill. 

The  faults  near  Mount  Oddie  and  Rushton  Hill,  which  sometimes  show  great 
displacement,  seem  to  cease  on  reaching  the  rhyolite,  like  the  faults  that  reach 
the  dacite  necks.  At  the  West  End  shaft  a  column  of  this  rhyolite  has  appar- 
ently ascended  the  fault  plane  which  runs  through  the  shaft.  Therefore  the 
rhyolite  is  younger  than  all  the  other  formations  excepting  the  Brougher  dacite, 
and  is  also  younger  than  the  faulting.  It  is  of  apparently  about  the  same  age 
as  the  Brougher  dacite,  and,  as  has  been  explained,  is  of  the  same  nature  and 
origin.  It  is  probable  that  the  rhyolite  and  the  siliceous  Brougher  dacite  vol- 
canoes were  contemporaneous,  and  that  adjacent  vents  gave  outlet  to  slightly 
differing  lavas.  The  petrologic  relationship  of  the  rhyolites  to  the  dacites  will 
presently  be  pointed  out. 

Mineral  coiiipositi{>n.~ Examined  microscopically,  the  rhyolite  shows  scattered 
jx>rphyritic  crystals  in  a  generally  fine-grained  microgranular  groundmass  con- 
sisting mainly  of  quartz  and  feldspar.  The  porphyritic  crystals  consist  of  quartz, 
orthoclase,  and  occasional  plagioclase,  one  determination  of  which  shows  andesine. 
Biotite  is  a  sparse  accessory.  Original  magnetite  and  sphene  have  been  noted. 
On  decomposition  the  rocks  yield  as  secondary  minerals  quartz  and  sericitc. 
sometimes  kaolin. 


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RHYOL1TES    AND    DAC1TES.  51 

LATEST   RHYOLITE  OR  DACITE. 

Location. — A  few  thin  sheets  of  glassy  rhyolite-dacite,  which  are  of  very 
little  importance,  do  not  clearly  seem  to  be  correlatahle  with  the  other  volcanic 
formations  described.  One  of  the  small  areas  of  this  lies  on  the  south  side  of 
Mount  Oddie.  This  rock  is  a  black,  very  glassy,  thin  flow,  overlying  a  coarse 
stratified  tuff  made  up  of  small  fragments  of  glass.  It  also  overlies  the  later 
andesite  in  such  a  way  as  to  indicate  that  the  tuffs  may  have  been  eroded  in 
places  from  the  andesites  before  the  glassy  sheet  was  poured  out. 

Similar  lava  occurs  around  the  base  of  Brougher  Mountain.  On  the  north 
side,  immediately  overling  the  tuff,  is  a  thin  bed  of  such  lava.  There  seems  to 
be  a  slight  unconformity  between  the  two.  Near  by,  the  glassy  lava  seems  to 
rest  on  the  Tonopah  glassy  rhyolite-dacite,  which  normally  underlies  the  Siebert 
tuffs,  suggesting  again  that  the  tuff  was  eroded  before  the  advent  of  the  lava. 

Age  and  origin. — These  flows  may  have  been  emitted  from  the  volcanoes  of 
Butler,  Brougher,  and  Oddie  mountains  during  their  earlier  history,  while  the 
cinder  cones  were  being  built  up,  or  as  the  writer  is  inclined  to  believe,  mainly 
during  their  later  history  and  so  subsequent  to  the  eruption  of  the  Brougher 
dacite.  They  are  not  observed  to  be  more  than  a  few  feet  thick.  In  places 
small  amounts  of  similar  lava  seem  to  have  ascended  as  dikes,  especiallv  along 
faults.  Where  it  occurs  as  dikes,  however,  it  may  be  difficult  to  distinguish  it 
from  some  of  the  glassy  rhyolite-dacite  lavas  of  other  periods. 

Mineral  composition. — Microscopically  the  lava  resembles  closelv  the  Tonopah 
rhyolite-dacite.  In  a  groundmass  of  brown  glass  there  are  porphyritic  crystals 
of  quartz,  orthoclase,  striated  feldspar,  and  biotite. 

SIEBERT   TOFF   (LAKE    BEDS). 
LACt'STRIXE   ORIGIN. 

The  white  stratified  tuffs  form  a  conspicuous  feature  "of  the  geology  near 
Tonopah.  As  a  rule  they  are  beautifully  and  uniformly  bedded,  and  composed  of 
well-assorted  material.  Where  beds  of  conglomerate  occur  the  pebbles  are  per- 
fectly rounded.  Since  these  sediments  do  not  vary  in  character  for  thicknesses 
of  several  hundred  feet,  it  is  plain  that  they  were  laid  down  in  a  large  body  of 
standing  water  that  lasted  for  a  considerable  length  of  time.  That  this  body 
was  a  lake  is  indicated  by  numerous  general  considerations  derived  from  the 
study  of  the  geology  of  the  surrounding  region  and  by  the  presence  of  numerous 
fresh-water  infusoria  in  some  of  the  strata.  In  contrast  to  the  general  regular 
stratification,  cross-bedded  strata  may  occasionally  be  found,  and  .at  one  place 
markings  like  those  made  by  rills  on  a  sandy  shore  were  noted.  These  are 
probably  shore  and  delta  features. 


52  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

Size  of  the  lake. — The  quantity  of  sediment  which  accumulated  in  this  lake 
shows  that  it  was  deep,  and  if  it  had  a  proportionate  areal  extent  it  must  have  been 
a  very  important  geographic  feature,  of  which  only  a  very  small  part  was  included 
in  the  area  mapped. 

Origin  of  lak<-  Ixmht.  —  The  lake  came  into  existence  at  the  close  of  the  most 
active  period  of  the  Tonopah  rhyolite-dacite  eruptions.  These  lavas,  as  well  as  those 
of  the  preceding  Fraction  dacite-breccia  eruptions,  were  poured  out  on  a  land  sur- 
face. The  formation  of  the  lake  was  due  to  a  depression  of  the  crust,  forming  an 
inclosed  basin,  or  to  a  climatic  change  with  increased  rainfall,  or  to  both  com- 
bined. It  is  at  least  certain  that  there  was  such  an  inclosed  basin,  and  while 
it  may  have  been  due  to  unknown  causes,  a  hypothesis  to  account  for  it  is 
suggested. 

The  extensive,  active,  and  long-continued  dacitic  eruptions,  which  are  attested 
by  the  Heller  dacite,  the  Fraction  dacite  breccia,  and  the  Tonopah  glassy  rhyolite- 
dacite  not  only  poured  out  or  showered  upon  the  surface  a  great  bulk  of  lava, 
but  emitted  an  enormous  volume  of  gas  and  steam,  which  mingled  with  the 
atmosphere.  At  the  close  of  the  active  eruptions  there  ensued  a  period  of  com- 
parative rest,  as  is  indicated  by  the  presence  of  fine-grained  and  undisturbed 
white  tuffs,  which  were  deposited  for  the  most  part  slowly.  As  the  incompletely 
occupied  spaces  left  by  the  violent  eruptions  were  filled  the  crust  subsided  of 
its  own  weight  and  the  basin  was  formed.  That  such  collapse  occurs  around 
centers  of  volcanism,  consequent  on  the  relief  obtained  by  outbreaks,  has  been 
proved  by  European  geologists. 

Sir  Archibald  Geikie.  in  his  study  of  the  ancient  volcanic  rocks  of  Great 
Britain,  refers  to  the  plateau  of  Antrim  in  the  north  of  Ireland,  as  follows: 

.  .  .  Hence  the  original  area  over  which  the  iron  ore  and  its  accompanying 
tuffs  and  clays  were  laid  down  can  hardly  have  been  less  than  1,000  square  miles. 
This  extensive  tract  was  evidently  the  site  of  a  lake  during  the  volcanic  period, 
formed  by  a  subsidence  of  the  floor  of  the  lower  basalts.  .  .  .  For  a  long  time 
quiet  sedimentation  went  on  in  this  lake,  the  only  sign  of  volcanic  energy  during 
that  time  being  the  dust  and  stones  that  were  thrown  out  and  fell  over  the  water 
basin  or  were  washed  into  it  by  rains  from  the  cones  of  the  lava  slopes  around. 

It  may  here  be  remarked  that  the  tendency  to  subsidence  in  the  Antrim  plateau 
seems  to  have  characterized  this  region  since  an  early  part  of  the  volcanic  period. 
The  lake  in  which  the  deposits  now  described  accumulated  was  entirely  effaced  and 
overspread  by  the  thick  group  of  upper  basalts.  Hut  long  after  the  eruptions  had 
ceased  a  renewed  sinking  of  the  ground  gave  rise  to  the  sheet  of  water  which  now 
forms  Lough  Neagh." 

Lough  Neagh,  which  occupies  the  deepest  part  of  this  hollow  and  covers  about 
one-eighth  of  the  whole  area  of  subsidence,  is  the  largest  sheet  of  fresh  water  in  the 
British  Isles.* 


«  A  m-i.-nt  Vulcanoeo  of  Great  Britain,  vol.  '2.  p.  21*.  '•  Op.  clt.,  p.  1 1.-. 


LAKE    BEDS. 


53 


We  may  conceive  that  after  the  cessation  of  the  outflows  of  basalt  the  territory 
overlying  the  lava  reservoir  that  had  been  emptied  would  tend  to  subside,  partly  by 
ruptures  of  the  crust,  producing  faults,  and  partly  by  a  downward  movement  of  a 
more  general  kind. " 

The  same  writer  remarks,  in  his  summary  of  observations:  * 

There  seems  to  have  been  commonly  a  contraction  and  subsidence  of  the 
material  in  the  vents,  with  a  consequent  dragging  down  or  sagging  of  the  rocks 
immediately  outside,  which  are  thus  made  to  plunge  steeply  toward  the  necks. 

Within  the  area  shown  on  the  Tonopah  map  a  similar  subsidence,  due  beyond 
question  to  the  causes  mentioned,  has  been  proved  by  the  writer  to  have  followed 
the  dacite  outbreak  which  brought  the  formation  of  the  tutf  and  the  lake  period 
to  a  close  (p.  47). 


NW 


SE 


FIG.  9.— Vertical  cross  section  of  southeast  side  of  Siebert  Mountain,  showing  relations  of  Siebert  tuffs  (lake  beds),  basaltic 
flow  and  agglomerates,  and  Brougher  dacite.  a,  finely  stratified  Siebert  tuffs  with  occasional  layers  of  rounded  pumice 
fragments  or  waterworn  lava;  b,  basaltic  agglomerate  with  bombs,  capped  by  solid  basalt;  c.  basalt;  d,  Brougher 
dacite,  intrusive  neck,  d',  glassy  marginal  facies  of  dacite. 

THICKNESS   OF   SEDIMENTS. 

On  account  of  the  complex  faulting  of  the  district  the  maximum  thickness  of 
the  Siebert  tuffs  can  not  be  given.  On  the  east  slope  of  Siebert  Mountain 
(PI.  IX,  ^1),  however,  an  unbroken  section  about  600  feet  in  thickness  is  exposed 
(fig.  9).  As  neither  the  bottom  nor  the  top  was  seen,  it  is  likely  the  maximum 
is  much  more  than  600  feet. 

CONDITIONS   DURING    DEPOSITION. 

The  Siebert  tuffs  rest  sometimes  on  the  earlier  andesite,  as  in  the  Tonopah  and 
California  shaft;  on  the  later  andesite,  as  southwest  of  Mount  Oddie;  or  more  often 
on  the  closely  connected  Fraction  dacite  breccia  and  the  Tonopah  rhyolite-dacite, 


a  Ancient  Volcanoes  of  Great  Britain,  vol.  2,  p.  460. 


t>Op.  cit.,  p.  473. 


54  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 

as  is  usually  the  case  in  the  southern  half  of  the  area  mapped.  These  facts  show- 
that  before  the  deposition  of  the  sediments  considerable  active  erosion  stripped 
off  the  debris  of  the  earlier  dacite-rhyolite  eruptions  and  bared  the  underlying 
andesites.  It  is  not  unlikely,  however,  that  the  land  which  adjoined  the  lake  and 
which  contributed  the  sediment  was  vigorously  worn  away,  and  that  the  sediments 
were  extended  over  this  eroded  region  as  a  result  of  a  rise  in  the  lake  or  of  a 
shifting  of  its  boundaries  due  to  crustal  movements.  This  idea  is  strengthened 
by  the  fact  that  a  careful  macroscopic  and  microscopic  study  of  the  materials  in 
the  tuffs  proves  that  they  were  derived  mainly  from  the  erosion  of  the  glassy 
dacites  and  rhyolites.  Pebbles  in  the  tuffs,  besides  those  of  the  rock,s  just  men- 
tioned, are  frequently  of  the  later  andesite,  well  rounded. 

EXPLOSIVE    ERUPTIONS    OF   THE    LAK        PERIOD. 

It  is  probable  that  the  quiet  of  the  lake's  existence  was  occasionally  slightly 
disturbed  by  small  local  eruptions  of  rhyolitie  or  dacitic  material.  Stratified 
beds  composed  of  rounded  waterworn  pumice  fragments  are  sometimes  found 
between  fine-grained  strata.  The  imperfect  bedding  shows  that  they  were 
deposited  more  hastily  than  most  of  the  strata,  and  each  bed  probabty  represents 
an  explosive  eruption.  Sometimes  angular  fragments  of  obsidian  occur  in  the 
pumice.  Moreover,  thin  sheets  of  Tonopah  rhyolite-dacite,  similar  to  the  main 
masses,  are  sometimes  found  within  the  tuff  series. 


UPLIFT  TERMINATING    LAKE   PERIOD. 


At  one  point  on  the  northeast  side  of  Siebert  Mountain  the  tuff  at  its  contact 
with  the  Siebert  dacite  body,  which  is  here  intrusive,  contains  a  conglomerate 
f  xmi  which  may  be  made  significant  inferences  as  to  the  conditions  prevailing  at 
the  time  of  its  formation.  This  conglomerate  is  made  up  of  rounded  pebbles  up 
to  4  inches  in  diameter,  most  of  which  are  composed  of  the  Tonopah  rhyolite-dacite, 
but  some  are  of  later  andesite.  In  it  was  found  a  fragment  of  silicified  wood 
over  a  foot  long.  This  conglomerate  is  exposed  for  only  about  50  yards.  It  dips 
with  the  inclined  tuffs,  but  is  not  continuous;  in  fact,  it  occupies  a  channel  in 
the  tuffs.  The  change  between  the  tuff  and  the  conglomerate  is  abrupt  and 
complete,  indicating  a  sudden  change  of  conditions.  All  this  suggests  that  these 
pebbles  are  old  river  gravels.  If  this  is  true,  the  tuffs  were  uplifted  at  the  close 
of  the  lake  period  and  became  land.  Immediately  thereafter  important  outbreaks 
of  lava  occurred,  and  the  hypothesis  may  be  formulated  that  the  accumulation  of 
the  lava  beneath  tin-  future  vents  produced  the  uplift.  A  river,  probably  flowing 
from  the  north  (where  the  later  andesite  is  now  and  was  then  exposed),  brought 
down  the  pebbles  to  this  bed.  That  the  banks  of  the  stream  were  wooded  is 
shown  by  the  now  silicitied  fragment. 


KrvV'W 
K/X^Jt' 


a  • 

r . .:  \  - '  \   ' 


3    (0  > 

a  CD  > 


BASALT.  55 


BASALTIC   ERfPTIOXS. 


The  conditions  thus  suggested  could  not  have  lasted  for  a  long  time,  for  at  a 
short  distance  from  the  conglomerate,  at  about  the  same  horizon  (on  the  east  side  of 
Siebert  Mountain  near  the  summit),  the  white  tuffs  are  overlain  by  l>eds  of  yellow 
pumice  breccia,  full  of  fragments  of  black,  slaggy  basalt,  a  rock  not  known  to  have 
been  previously  erupted.  Small  hollow  spheres  of  pumice  (lava  bubbles)  are 
present.  Some  layers  are  made  up  entirely  of  large,  angular  fragments  of  scori- 
aceous  basalt.  Over  this  lies  a  bed  of  black  basalt  40  or  50  feet  thick.  This  rude 
accumulation  of  pumice  and  scoria;  appears  to  lie  unconformably  on  the  tuffs,  for  it 
is  nearly  horizontal,  while  the  tuffs  have  a  decided  dip  to  the  west;  and  the  same 
breccia  appears  at  several  other  points  on  the  mountain  in  contact  with  different 
horizons  of  the  tuffs.  The  uplifted  tuff's  of  the  same  age  as  the  river  conglomerate 
were  probably  tilted  bodily  to  the  west  by  a  continuation  of  the  disturbing  uplift, 
and  after  this  tilting  new  volcanic  vents  were  opened  and  there  occurred  a  violent 
explosion  which  scattered  a  relatively  slight  amount  of  basaltic  material.  This 
explosion  was  followed  in  the  neighborhood  of  Siebert  Mountain  b\-  the  welling 
out  of  a  thin  sheet  of  slaggy  basalt.  On  Brougher  Mountain  also  a  volcanic  breccia 
overlies  the  tuffs,  but  here  no  basalt  is  exposed. 


KKdlOXAL    TIl.TIXli    ACCOMPANYING    UPLIFT. 


The  uplift  which  preceded  the  explosions  was  not  local.  The  westward  dip  of 
the  tuffs  on  Siebert  Mountain  is  not  essentially  different  from  their  general  attitude 
wherever  found  in  the  area  mapped.  There  is  a  notably  persistent  north-south 
strike,  and  a  westward  dip  averaging  perhaps  20°,  independent  of  local  phenomena 
These  local  phenomena  bring  about  variations  in  the  attitude;  for  example,  near  the 
great  Butler  Mountain  neck,  where,  as  will  be  presently  explained,  the  rocks  have 
been  faulted  and  dragged  down  at  the  contact,  there  are  places  where  the  tuff  is 
locally  folded  so  that  it  dips  toward  the  mountain. 

HASALT. 
LOCATION. 

Basalt  in  place  occurs  in  only  one  small  area  within  the  district  mapped— 
near  the  top  of  Siebert  Mountain  (PI.  XI),  although  it  was  observed  in  three  other 
places,  close  to  the  area.  Near  Tonopah,  on  the  road  from  Sodaville,  low  hills  of 
vesicular  lava  stand  on  the  edge  of  the  wash-covered  desert  valley.  This  lava  is 
an  augite-olivine-basalt,  containing  augite  and  reddish  altered  olivine  in  a  micro- 
litic  groundmass  consisting  of  feldspar,  augite,  olivine,  and  magnetite. 

The  top  of  a  broad,  black  mountain  just  north  of  Ararat  Mountain  is  alos 
covered  with  basalt  of  the  kind  just  mentioned.  A  determination  of  one  of  the 
feldspars  here  showed  anorthite.  Similar  lava  forms  the  hill  east  of  Golden 
Mountain  and  overlies  the  Fraction  dacite  breccia. 


56  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,   NEVADA. 

RELATIONS   AND  COMPOSITION   OK   BASALT  OF  SIEBERT    MOUNTAIN. 

Particulars  concerning  the  age  of  these  two  occurrences  can  not  be  given, 
but  the  basalt  on  Siebert  Mountain  has  been  more  carefully  studied  than  that 
north  of  Ararat  Mountain.  The  white  tuffs  which  make  up  the  bulk  of  the 
mountain  are  overlain  by  a  breccia  of  yellow  pumice  containing  fragments  of 
scoriaceous  basalt.  This  breccia  probably  rests  unconformably  on  the  tuffs,  which 
are  tilted,  and  is  overlain  by  a  flow  of  vesicular  basalt  40  or  50  feet  thick.  This 
flow  extends  southwest  of  the  mountain,  beyond  the  limits  of  the  area  mapped. 
Basalt  inclusions  occur  also  in  the  Brougher  dacites  (see  p.  45). 

Under  the  microscope  this  basalt  shows  small  porphyritic  crystals  in  a  fine 
holocrystalline  groundnrass  consisting  chiefly  of  feldspar  and  augite.     The  porphy 
ritic   crystals  are   predominating   pale -green   augite,  brown  hornblende  partly  or 
wholly  altered  to  iron  oxide  by  magmatic  reactions,  and  feldspar. 

AGE. 

This  basalt  overlies  the  tuffs  unconformably,  so  it  must  have  been  erupted 
subsequent  to  the  tilting.  It  and  the  tuff  are  intruded  by  the  neck  of  dacite 
which  outcrops  all  over  the  summit,  and  which  by  its  resistance  to  erosion  has 
created  the  mountain.  On  the  east  side  of  the  mountain  a  fault  has  displaced 
the  basalt  flow  and  the  tuff,  but  has  not  affected  the  dacite  (Pis.  X,  XI). 

CHEMICAL   COMPOSITION    OF   LAVAS.' 

For  the  purpose  of  comparison  the  analyses  of  the  fresh  rocks  of  the  district 
have  been  assembled  in  the  accompanying  table.  To  represent  the  earlier  andesite, 
since  no  fresh  specimen  is  available,  an  ideal  type  of  hornblende-mica-andesite 
(p.  217)  has  been  substituted,  practically  identical  with  the  analyses  of  the  least 
altered  earlier  andesite  except  as  to  the  amount  of  silica.  The  knowledge  obtained 
by  these  analyses,  though  valuable,  is  only  fragmentary,  and  more  investigation 
would  certainly  show  a  greater  variation. 

TRANSITIONS   IN   SILICA   CONTENT. 

The  analyses  have  been  arranged  according  to  their  silica  content,  which  shows 
the  following  differences:  Between  the  basalt  and  the  later  andesite  about  3  per 
cent;  between  the  earlier  and  the  later  andesites  approximately  6  per  cent;  between 
the  earlier  andesite  and  the  least  siliceous  dacite  about  6  per  cent;  and  between 
this  dacite  and  the  siliceous  rhyolite  about  5  per  cent.  This  transition  of  silica 
content  is,  then,  fairly  equable,  but  considering  the  analyses  as  a  whole  there  is 
a  marked  break  between  4  and  5 — that  is,  between  the  andesite-basalts  on  the  one 
hand  and  the  dacite-rhyolites  on  the  othe.r.  The  same  break  is  shown,  even 
more  plainly,  in  the  iron  and  magnesia  content,  and,  to  a  less  degree,  in  the  lime 
percentage. 


'jj: 


\ 


\. 


0 

z 


a. 


z 
o 


11. 

O 
IT 
CL 


a 

3 


O 
O 

o 
o 

UJ 

O 

to 

j 


CHEMICAL    COMPOSITION    OF    LAVAS. 


57 


A  rather  characteristic  difference  between  the  dacites  and  the  rhyolites  is  the 
predominance  of  potash  over  soda  in  the  latter;  and  in  this  particular  the  inter- 
mediate character  of  the  Tonopah  rhyolite-dacite  is  also  seen. 

Analyses  of  Tonopah 


i. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

10. 

SiO2 

53.94 

56.26 

57.51 

62.  16 

71.71 

72.31 

73  00 

75  56 

75  66 

76  57 

ALO,-- 

16.18 

16.  55 

16.45 

14.00 

13.79 

FesO,  

5.56 

3.20 

3.27 

1.06 

1.54 

FeO  

1.17 

2.02 

2.71 

.51 

.26 

MgO  .. 

2.78 

2.30 

2.20 

.43 

.56 

CaO 

7.32 

5.07 

6.06 

4.  13 

2  25 

1  08 

1  55 

1  16 

47 

Na,O 

3.89 

3  25 

2  76 

4  07 

3  21 

2  56 

3  50 

4  20 

1  70 

96 

K2O     .. 

2.09 

3.43 

2.81 

3.45 

4.41 

4.66 

4  71 

4.50 

4  94 

5  81 

H,O- 

2.07 

1.45 

44 

H2O+  

2.61 

2.56 

1.15 

1.38 

TiO, 

.73 

.80 

.28 

.27 

P.(X-- 

.32 

.30 

.07 

.07 

99.43 

98.32 

99.59 

99.75 

97.10 

a  Analyses  1,  7,  8,  9.  and  10  are  by  Dr.  E.  T.  Allen;  analysis  2  by  Dr.  \V.  F.  Hillebraiul:  analyses  3,  5,  and  6  by  Mr. 
George  Steiger. 

1.  Basalt,  Siebert  Mountain  (specimen  168).     This  basalt  is  not  typical  chemically,  containing 
only   2.37   less   silica   than   the   andesite,    analysis   Xo.    2.     It   appears   to   fall,    more  accurately 
considered,   into   the  group  intermediate   between   the  basalts  and   the  andesites,  for  which  the 
writer  has  proposed  the  name  tileutite.     For  the  same  reasons  that  are  given  later  for  not  using  the 
term  latite,  however,  the  name  basalt  will  be  retained. 

2.  Augite-biotite-andesite  (later  andesite),  Halifax  shaft  (specimen  349). 

3.  Augite-biotite-andesite  (later  andesite),  Mizpah  Extension  shaft  (specimen  225). 

4.  Hornblende-biotite-andesite  (earlier  andesite).     (See  p.  217.) 

5.  Mountain  dacite,  Brougher  Mountain  (specimen  359). 

6.  Glassy  Tonopah  rhyolite-dacite,  2,700  feet  north  of  King  Tonopah  shaft  (specimen  661). 

7.  Mountain  dacite,  Butler  Mountain  (specimen  368). 

8.  Mountain  dacite,  Golden  Mountain  (specimen  388). 

9.  Rhyolite,  Belmont  shaft,  Rushton  Hill  (specimen  376). 
10.  Rhyolite,  G.  &  H.  tunnel,  Mount  Oddie  (specimen  337). 

CHEMICAL  COMPOSITION  OF  THE   DAC1TE-RHYOLITE  SERIES. 

Differences  and  relations. — The  volcanic  rocks  which  have  been  described  as 
dacites  and  rhyolites  often  differ  markedl}'  in  composition  as  well  as  in  age.  For 
example,  the  rock  of  Brougher  and  Butler  mountains  is  quite  different  from  that 
of  Mount  Oddie,  as  is  evident  to  every  one,  be  he  geologist  or  not.  Yet  the  two 
rocks  are  closely  related  and  there  are  transitions  between  them,  as  represented, 
for  example,  in  the  rock  in  parts  of  Golden  Mountain. 


58 


GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 


Comparison  with  Eureka  and  Washoe  dacite*  and  rhyolites. — It  is  important 
to  ascertain  the  position  of  the  Tonopah  rocks  with  reference  to  (1)  dacites  and 
rhyolites  which  have  been  described  by  Becker  and  by  Hague  and  Iddings  from  the 
neighboring  and  closely  related  districts  of  Washoe  and  Eureka  (for  these  districts 
and  their  rocks  will  often  be  compared  with  Tonopah  in  the  present  report),  and 
(2)  to  the  system  of  igneous  rocks  as  a  whole.  The  comparison  with  the  Washoe 
and  Eureka  rocks  is  shown  by  the  following  analyses,"  which  are  arranged 
according  to  silica  content. 

Analyses  of  rlacite  and  rhi/olite  from   Tonopah  ami  other  districts  in  Nevada. 


i. 

2.   i   3. 

4. 

5. 

6. 

7.      8. 

9- 

10. 

11. 

12. 

SiO2.... 
A1,OH... 

67.03 
16.27 

69.96  71.71 
15.79  14.00 

72.31 
13.  79 

73.00 

73.07 
11.18 

73.  09  73.  91 
14.47  15.29 

75.  56 

75.66 

75.69 
12.26 

76.57 

Fe,O, 

2.50   1.06 

1.54 

2.30 

FeO 

3.97 

51 

.26 

2.  99    .  89 

2.93 

MizO 

1  19 

.64     43 

56 

39 

CaO.... 

3.42 

1.73   2.25 

1.08 

1.55 

2.02 

1.13    .77 

1.16 

.47 

1.  13 

Na,O  -  .  . 
•K2O.... 
TiO, 

2.93 
3.96 

.58 

3.  80   3.  21 
4.12   4.41 
28 

2.56 
4.66 
.27 

3.50 
4.71 

1.19 
6.84 

2.77   3.62 
5.  07   4.  79 

4.20 
4.50 

1.70 
4.94 

3.01 
4.74 

.96 
5.81 

PA  - 

.23 

07 

.07 

:   .07 

.06 

SO, 

i 

1.  Dacite,  Eureka,  Nev. 

2.  Dacite,  Washoe,  Nev. 

3.  Brougher  dacite,  Brougher  Mountain,  Tonopah  (specimen  359). 

4.  Tonopah  rhyolite-dacite,  Tonopah  (specimen  661). 

5.  Brougher  dacite,  Butler  Mountain,  Tonopah  (specimen  368). 

6.  Rhyolite,  Washoe,  Nev. 

7.  Rhyolite,  Eureka,  Nev. 

8.  Rhyolite,  Eureka,  Nev. 

9.  Brougher  dacite,  Golden  Mountain,  Tonopah  (specimen  388). 

10.  Rhyolite,  Rushton  Hill,  Tonopah  (specimen  376). 

11.  Rhyolite,  Eureka,  Nev. 

12.  Rhyolite,  Mount  Oddie,  Tonopah  (specimen  337). 

There  is  a  close  relation  between  Nos.  "2  and  3,  dacites  from  Washoe  and 
Tonopah  (Brougher  Mountain).  The.se  rocks  are  plainly  almost  identical,  and 
suggest  the  general  correlation  of  the  dacites  of  the  two  districts,  although  the 
high  silica  content  of  No.  i»,  dacite  from  Tonopah  (Golden  Mountain),  has  caused 
it  to  )>e  placed  in  the  table  l>etween  a  Eureka  rhyolite  and  a  Tonopah  rhyolitr. 

Retetitum  of  t lie  tit'in  dacite. — The  analyses  represent  a  series  of  closely  related 
rocks  which  show  a  transition  from  No.  1,  which  has  nearly  the  composition  of  an 


"The  Eureka  und  Waahoe  analvxes  are  taken  from  Mon.  I'.  8.  Geol.  Survey,  vol.  20.  pp.  261,  • 


CLASSIFICATION    OF    RHYOLITIC    BOCKS.  59 

andesite,  to  No.  12,  an  extremely  siliceous  and  potassic  rhyolite/'  Separation  of  this 
series  into  dacites  and  rhyolites  is  evidently  largely  arbitrary;  hut  the  dacites  and 
rhyolites  of  Tonopah  appear  to  be  roughly  comparable  to  those  of  Eureka  and  Washoe, 
and  as  they  are  on  the  whole  distinct  rocks  (in  spite  of  the  transitions)  it  is  desirable 
to  have  separate  field  names  for  them.  For  this  reason  it  seems  advisable  to  the 
writer  to  retain  the  field  name  dacite  for  the  less  siliceous  and  alkalic  of  the  dacite- 
rhyolite  rocks  at  Tonopah.6 

Rhyolitic  nature  of  loth  dacites  and  rkyoJite*. — To  determine  the  position  of  the 
Tonopah  dacite-rhyolites  in  the  system  of  igneous  rocks  the  writer  has  compared 
their  analyses  with  similar  analyses.  As  almost  all  comparable  rocks  have  been 
classed  as  rhyolites,  this  designation  would  apply  to  these  rocks,  and  there  would  be 
no  distinction  between  the  white  siliceous  rock  of  Mount  Oddie  and  the  darker  rock 
of  Brougher  Mountain.  If  the  region  had  been  mapped  without  strict  accuracy  and 
detail,  therefore  all  these  phases  would  probably  have  been  included  together  and 
mapped  collectively  as  rhyolites,  and  the  significance  of  their  relations  would  have 
been  lost  sight  of. 

Determination  according  to  a  quantitative  classification. — The  word  rhvolite  is 
part  of  the  old-established  classification,  and  its  meaning  is  indefinite  and  inexact. 
Undoubtedly  the  most  notable  attempt  at  an  exact  classification  of  igneous  rocks  is 
that  recently  made  by  Cross,  Iddings,  Pirsson,  and  Washington/'  Their  own  char- 
acterization of  the  sj'stem  is  as  follows: 

"This  system  is  a  chemico-mineralogical  one.  All  igneous  rocks  are  classified 
on  the  basis  of  their  chemical  composition,  and  all  rocks  of  like  chemical  composi- 
tion are  grouped  together.  The  definition  of  the  chemical  composition  of  a  rock  is 
expressed  in  terms  of  certain  minerals  capable  of  crystallizing  from  a  magma  of  the 
given  chemical  composition,  and  the  expression  is  quantitative."'' 

a  Such  rooks  have  been  called  tordrillite  by  the  writer.    Am.  Geologist,  vol.  25,  p.  230. 

("Since  the  classic  work  done  in  Nevada  by  Zirkel,  Hague  and  Iddings,  Becker,  and  others,  some  further  division  in 
petrographic  nomenclature  has  been  made  in  rocks  similar  to  those  which  they  studied.  Brogger  has  given  the  name 
monzonite  to  granular  rocks  occupying  an  intermediate  chemical  position  between  granites  and  diorites.  This  group 
therefore  is  made  up  of  rocks  which  previously  were  classified  either  as  granitesor  diorites.  Dr.  F.  L.  Ransome  has  followed 
out  this  idea  and  assigned  a  special  name— latite— to  extrusive  rocks  having  a  monzonitic  composition.  This  new  division  is 
made  upof  rocks  previously  classified  as  rhyolites,  dacites,  and  andesites  The  Sierra  Nevada  volcanic  province  whose  latites 
were  described  by  Dr  Ransome  is  probably  part  of  the  same  petrographic  province  as  that  in  which  Nevada  lies  (Spurr. 
3.  E.,  Jour.  Geol.,  vol.  8,  No  7,  p.  638).  Latites,  indeed,  are  abundant  in  Nevada,  and  have  there  been  described  by  the 
writer;  and  mouzonitesare  also  present  (Spurr,  J.  E  .  Bull  U.S. Geol  Survey  No. 208, pp.53, 59, 73,  92, 108. 118, 122.126.  Hl.lstij. 
The  latites  correspond  to  a  part  of  the  dacites  and  andesites  described  by  the  earlier  investigators  in  the  region,  as  previously 
pointed  out  by  the  writer  (Spurr,  J.  E.,  Jour.  Geol.,  vol  8.  no.  7,  p.  643).  Thus  a  number  of  the  dacite  and  andesite  analyses 
given  for  the  Washoe  and  Eureka  rocks  would  to-day  be  doubtless  classified  as  latite  by  most  petrographers. 

Under  the  newer  nomenclature  and  subdivision,  therefore,  the  rhyolitic  series  at  Tonopah  would  pass  with  decreasing 
silica.increasing  lime,  and  attendant  changes  to&lntite  rather  than  a  docile,  and  this  is  theclassification  which  the  writer  would 
use  were  the  Tonopah  district  an  independent  problem.  Actually,  however,  the  correlation  of  these  Tonopah  lavas  with 
those  already  described  at  Washoe  and  Eureka,  as  well  as  other  parts  of  Nevada  (Spurr,  J.  E.,  Jour.  Geol.,  vol.  8,  no.  7,  pp. 
621-646),  is  a  highly  important  feature  of  the  investigation;  and  most  of  the  previous  work  on  this  region  has  been  stated 
simply  in  terms  of  basalt,  andesite,  dacite,  and  rhyolite.  Thus  the  writer  would  be  compelled  to  reorganize  completely  the 
literature  of  the  province  in  order  not  to  introduce  more  confusion  than  illumination,  and  this  task  he  does  not  at  present 
feel  able  or  anxious  to  undertake. 

<•  Quantitative  Classification  of  Igneous  Rocks,  1903. 

<*  Washington,  H.  S.,  Chemical  analyses  of  igneous  rocks:  Prof.  Paper  U.  S.  Geol.  Survey,  No.  14,  p.  47. 


60 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 


Rocks  of  different  mineralogical  but  similar  chemical  composition  are  riot  dis- 
tinguished, therefore  the  classification  is  one  of  magmas,  and  is  especially  valuable 
in  discussions  of  the  relation  of  magmas. 

The  Tonopah  dacite  and  rhyolite  analyses  (the  last  six  in  the  table  on  p.  57) 
were  classified  according  to  this  system.  The  results  are  as  follows: 

Position  of  Tonopah  rhyolites  and  (Incites  in  the  quantitative  classification. 


No.  anal, 
in  table 
on  p.  57. 

Speci- 
men 
No. 

, 
Field  name. 

Locality. 

Class. 

Order. 

Rang. 

Subrang. 

Name. 

5 

359 

Dacite 

Persalane. 

Quardofelic. 

Domalkalic. 

Sodipotassic  . 

Toscanose. 

368 

tain. 

do 

do  

do  

do  

Do. 

g 

388 

dacite. 
do 

...do     .. 

do  

...do... 

do  

Do. 

6 

661 

1  mile  N.  of  King 

do  

Quarfelic  .  .  . 

do  

....do  

Tehamose. 

9 

376 

rhvolite- 
da'cite. 

Rhvolite    .. 

Tonopah  shaft. 
Belmont  shaft  

do  

do  

do  

l>opotassic.  .. 

Magdeburgose. 

10  

337 

do  

G.  and  H.  tunnel, 
Mount  Oddie. 

do.... 

do  

do  

«lo  

Do. 

Thus  it  is  seen  that  all  the  Brougher  dacite  falls  under  one  subrang,  toscanose; 
the  rhyolite  falls  under  a  quite  distinct  order,  rang,  and  subrang,  magdeburgose; 
while  the  Tonopah  rhyolite-dacite  is  in  the  same  order  as  the  rhyolite  (though  nearly 
in  the  same  order  as  the  Brougher  dacite)  and  otherwise  like  the  Brougher  dacite: 
so  that  it  falls  into  the  subrang  tehamose. 

These  divisions  correspond  to  the  natural  divisions;  and  the  classification  is 
evidently  in  this  case  a  suitable  one. 

It  may  be  added  that  the  dacite  from  Washoe,  Nev.  (analysis  No.  2  in  table  on 
page  58),  is  classified  by  Washington"  as  toscanose,  like  the  Tonopah  dacites,  and 
rhyolite  from  Eureka,  Nev.  (analysis  No.  7,  p.  58),  as  mihalose  (near  dellenose).* 
It  is  of  the  same  order  and  rang  as  the  Tonopah  rhyolite-dacite  of  Tonopah,  but 
of  a  dopotassic  subrang,  like  the  Tonopah  rhyolite,  and  is,  therefore,  intermediate 
between  these  two  Tonopah  rocks. 

Varying  composition  of  lavas  in  different  vents. — The  transition  phases  of  the 
dacite-rhyolite  are  not  limited  to  small  areas,  but  are  represented  by  large  masses; 
so  that  there  is  no  fixed  point,  either  theoretical!}'  or  in  the  field,  where  one  can  be 
separated  from  the  other.  Each  vent,  now  represented  by  a  more  or  less  separated 
and  isolated  volcanic  plug,  seems  to  have  ejected  nearly  homogeneous  lavas  that 
differed  slightly  in  composition  from  the  lavas  from  neighboring  vents.  Thus  the 
wilica  content  in  the  dacite-rhyolite  series  was  least  in  the  Brougher  Mountain  vent, 
and  increased  successive!}'  in  Butler  Mountain,  Golden  Mountain,  Rushton  Hill,  and 
Mount  Oddie.  The  difference  between  the  lava  of  Brougher  Mountain  and  that  of 
Mount  Oddie  is  very  considerable.  When  compared  with  the  Brougher  Mountain 

nOp.  clt.,  p.  167.  &0p.  cit.,  p.  181. 


DIFFERENTIATION    OF    LAVAS    FROM    A    UNIFORM    TYPE.  61 

lava,  the  lava  of  Mount  Oddie  shows  an  increase  of  4.86  per  cent  silica  and  of  1.41 
per  cent  potash;  and  a  decrease  of  2.25  per  cent  soda,  and  probably  2  per  cent  lime. 
The  course  followed  by  this  gradual  change  from  Brougher  Mountain  to  Mount 
Oddie  by  way  of  Butler  Mountain,  Golden  Mountain,  and  Kushton  Hill,  is  almost 
circular;  and  while  more  extended  knowledge  is  desirable,  it  has  probably  a  signifi- 
cance, for,  as  already  explained,  all  these  vents  belonged  to  the  same  period,  though 
they  were  not  necessarily  absolutely  contemporaneous.  They  may  well  have  been 
successive  centers  of  outbreak  in  the  order  given. 

THEORY   OF    DIFFERENTIATION   OF   TONOPAH    LAVAS   FROM   A    UNIFORM   TYPE. 

PSEUDOMORPHS    IX    RHVOLITE. 

Character  of  pseudomorphs. — The  description  of  the  first  specimen  of  rhyolite 
analyzed,  as  seen  under  the  microscope  (column  10  in  table  on  p.  58),  is  as  follows: 

Specimen  376,  from  Belmont  shaft,  50  feet  down.  This  rock  shows  to  the  naked 
eye  small  fresh  crystals  of  orthoclase  (sanidine),  quartz,  and  biotite  in  a  pinkish- 
white  groundmass.  Abundant  small,  dull-white  spots  often  have  crystalline  form, 
and  seem  to  play  the  part  of  phenocrysts. 

Under  the  microscope  the  rock  is  seen  to  be  fresh.  The  sanidine  shows 
sometimes  Carlsbad  twinning;  it  is  often  broken,  and  may  be  partly  resorbed  by 
the  magma.  The  quartz  is  frequently  in  dihexahedral  crystals,  rounded  and 
invaded  by  the  resorbing  magma.  The  biotite  is  fresh,  in  small  crystals,  and  in 
very  small  amount.  The  groundmass  is  a  fine  microgranular  aggregate  of  quartz 
and  feldspar. 

Some  of  the  dull-white  spots  noticed  in  the  hand  specimen  are  without 
crystal  outlines,  while  others  have  sharp  outlines.  Inspection  of  u  number  of 
longitudinal  and  cross  sections  leads  to  the  conclusion  that  the  forms  are  probably 
those  of  hornblende.  The  material,  however,  is  evidently  pseudomorphous,  for 
it  is  a  fine  transparent  aggregate  of  low  single  and  double  refraction,  which 
under  high  powers  is  seen  to  be  spherulitic.  It  separates  itself  from  the  rest  of 
the  groundmass  chiefly  by  its  greater  fineness.  In  several  cases  small  biotite 
crystals  were  observed  in  this  aggregate,  as  large  as  many  in  the  rest  of  the 
rock,  and  these  were  clustered  together  with  a  tendency  to  a  diverging  or  radial 
arrangement. 

The  description  of  the  second  specimen  of  rhyolite  analyzed  is  as  follows: 

Specimen  337,  from  face  of  G.  and  H.  Tunnel,  Mount  Oddie,  contains  larger 
phenocrysts  than  usual  of  quartz,  orthoclase,  and  a  little  biotite  in  a  fine  microgran- 
ular groundmass  of  quartz  and  orthoclase.  The  feldspar  is  glassy  and  fresh  sani- 
dine. The  biotite  contains  apatite  crystals,  which  are  clear,  not  smoky  like  those 
of  the  andesites. 


62  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

After  the  observations  made  on  the  areas  apparently  pseudomorphous  after 
hornblende  in  No.  376,  similar  areas  were  looked  for  in  this  rock.  They  were 
at  first  not  evident,  but  some  definite  though  irregular  areas  of  a  fine  aggregate 
similar  to  the  pseudomorphs  referred  to  were  found.  On  close  observation, 
however,  faint  but  distinct  crystal  forms  shaped  like  those  of  No.  337  were  dis- 
tinguished. The  area  occupied  by  these  forms  is  surrounded  by  a  border  of 
similar  fine  aggregate,  running  irregularly  off  into  the  rest  of  the  rock,  which 
so  obscures  the  crystal-like  outline  that  it  would  not  have  been  detected  save  for 
the  observations  made  on  No.  337.  This  aggregate  is  somewhat  coarser  than  in 
No.  337,  and  its  nature  can  be  determined.  It  is  semispherulitic  and  semigranular, 
and  differs  from  the  rest  of  the  groundmass  only  in  being  slightly  finer  grained 
and  containing  a  little  more  biotite.  It  is  a  fine  mixture  of  quartz,  orthocluse 
(sanidine),  and  biotite.  Very  small  idiomorphic  crystals,  or  phenocrysts,  of  sani- 
dine  form  part  of  the  aggregate.  It  thus  appears  that  the  original  hornblende 
(in  part  pyroxene?)  substance  has  been  replaced  by  rhyolitic  material. 

Jfugmativ  or'ujin  of  psevdomorphs. — Since  these  pseudomorphs  in  No.  337  are 
often  in  direct  contact  with  perfectly  glassy  sanidine,  they  must  be  of  magmatic 
origin  and  must  have  been  formed  before  or  during  the  consolidation  of  the 
rhyolite.  It  is  probable  that  they  represent  hornblende,  which  was  an  earlv 
mineral  to  crystallize  and  was  afterwards  decomposed  by  the  siliceous  magma 
and  pseudomorphosed  to  biotite  and  the  fine  aggregate.  The  process  was  plainly 
a  partial  replacement  of  some  material  by  others,  for  no  mineral  containing  lime 
in  any  quantity  resulted.  Indeed,  it  is  somewhat  difficult  to  determine  where 
the  lime  went  to,  for  the  analysis  of  the  rock  shows  only  so  much  lime  as  is 
commonly  contained  in  orthoclase.  It  seems  difficult  to  explain  .such  a  process 
as  this  without  supposing  a  chemical  change  in  the  magma. 


HOUXHLEXDE    IX    TOXOI'AH    LAVAS. 


No  hornblende  or  augite  has  been  found  in  the  white  Tonopah  rhyolites. 
In  the  Tonopah  rhyolite-dacite  no  fresh  hornblende  was  seen,  but  there  was 
found  in  it  one  pseudomorph  after  hornblende,  marked  by  crystals  of  specular 
iron,  the  hornblende  having  been  resorbed  by  the  magma  (p.  41).  In  the  glassy 
Tonopah  rhyolite-dacite  also  only  one  small  crystal  of  augite  was  seen  out  of  very 
many  thin  sections  examined.  In  the  Brougher  dacite  hornblende  is  rare,  but  has 
been  occasionally  found.  A  specimen  of  dacite  from  Golden  Mountain,  at  a  point 
south  of  the  top  of  Mount  Oddie,  showed  a  single  fresh  hornblende  crystal.  This 
Golden  Mountain  dacite  is,  as  shown  by  the  analyses  (p.  58),  closely  related  to  the 
near-by  Oddie  rhyolite,  so  that,  as  has  already  been  mentioned,  the  two  must  be 
considered  as  variations  of  a  single  magma.  Augite  is  rare  in  the  Hrougher  dacite. 


DIFFERENTIATION    OF    LAVAS    FROM    A    UNIFORM    TYPE.  H3 

but  is  occasionally  met,  more  often  than  hornblende.  In  all  the  dacites  and 
rhyolites,,  the  dark  mineral  is  almost  exclusively  biotite.  The  earlier  andesite,  on 
the  other  hand,  contains  abundantly  both  hornblende  and  biotite,  with  some  augite, 
while  the  later  andesite  contains  much  augite  and  biotite,  with  some  hornblende. 
The  basalt,  again,  contains  abundantly  both  augite  and  hornblende,  the  latter  often 
partly  or  wholly  resorbed  by  magmatic  action  and  pseudomorphosed  into  aggre- 
gates of  iron-oxide  crystals.  No  biotite  is  present.  The  presence,  or  evidence  of 
the  former  presence,  of  hornblende  is  thus  shown  in  nearly  all  the  Tonopah 
volcanics,  from  the  very  siliceous  to  the  very  basic,  and  emphasizes  their  consan- 
guinity. But  the  number  of  hornblende  crystals  (it  is  possible  that  some  of  these 
pseudomorphs  were  also  after  augite)  indicated  by  the  pseudomorphs  above 
described  as  having  been  originally  present  in  the  unconsolidated  rhyolitic  magma 
is  large,  being  equaled  only  in  the  earlier  andesites  and  the  basalts. 

DERIVATION    OF    KHYOLITE    AND    HASA  l,T    FROM    INTERMEDIATE    MAKMA. 

Statement  of  theory. — The  Oddie  rhyolite  is  considerably  separated  from  the 
earlier  andesites  in  age,  while  it  was  nearly  contemporaneous  with  the  basalt  of 
Siebert  Mountain.  In  this  basalt  the  partly  corroded  and  pseudomorphosed 
hornblende  crystals  indicate  that  the  hornblende  was  an  earlier  crystallization,  not 
entirety  stable  under  the  later  conditions  of  the  magma,  which  produced  naturally 
iittgite.  That  is  to  say,  both  the  highly  siliceous  rhyolitic  magma  and  the  basic 
basaltic  magma  developed,  as  first  mineral,  hornblende,  which  in  each  case  was 
unsuited  to  later  conditions;  the  magma  of  the  rhyolite  became  more  siliceous  and 
alkaline,  so  that  biotite  was  formed  as  the  dark  mineral,  and  that  only  sparingly; 
the  magma  of  the  basalt  became  more  basic  and  calcareous,  so  that  abundant 
augite  was  formed.  If  this  is  so,  then  these  two  magmas  at  the  time  of  the  Hrst 
hornblende  crystallization  must  have  been  more  nearly  intermediate  in  nature  and 
approached  each  other  more  closely;  and  as  they  were  erupted  at  nearly  the 
same  locality  they  may  possibly  have  been  nearly  or  quite  the  same.  Such  a 
common  intermediate  magma  might  have  a  composition  like  that,  for  example, 
of  an  andesite.  These  considerations  would  harmonize  with  the  hypothesis  that 
the  writer  adopted  several  years  ago,  that  the  contemporaneous  "complemen- 
tary" rhyolites  and  basalts  of  the  Great  Basin  region  were  the  results  of  the 
splitting  up  of  a  magma  of  intermediate  composition. a 

jRhy  elite-bandit  differentiation  theory  tested  by  analyses. — Complete  analyses  of  the 
basalt  and  of  the  Oddie  rhyolite  were,  unfortunately,  not  made;  one  partial  analysis 
of  each  shows  the  relative  amounts  of  silica,  lime,  and  the  alkalies.  These  analyses 
may  be  compared  in  considering  the  theory  that  the  basalt  and  the  rhyolite  are  the 

"Succession  and  relation  of  the  lavas  of  the  Great  Basin:  Jour.  Geol.,  vol.  s,  pp.  621-616. 


64 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 


two  parts  of  an  original  andesitic  magma.  The  average  of  the  anatyses  of  these 
rocks  resembles  the  analysis  of  the  type  of  hornblende-mica-andesite,  taken  as  a 
standard  in  default  of  any  fresh  andesite  of  this  kind  in  Tonopah  (p.  217). 

Comparison  of  the  means  of  the  analyzes  of  rhyolltic  and  basaltic  rocks  of  Tonopah  with  those  of  andesitic  rocks. 


1  (376). 

2  (168). 

I 

3. 

4. 

5. 

6. 

SiO2 

75.66 

53.  94 

64.80 

62.16 

65.  13 

65  68 

CaO. 

.47 

7.32 

3.89  ' 

4.13 

3.62 

3.50 

NajO 

1  70 

3.89 

2.79  ! 

4.07 

2  93 

3  20 

K2O 

4.94 

2.09 

3.51 

3.45 

3.96 

3  37 

1 

1.  Siliceous  rhyolite,  Belinont  shaft. 

2.  Basalt,  Mount  Siebert. 

3.  Average  of  1  and  2. 

4.  Average  type  of  andesite. 

5.  Andesitic  pearlite,  Eureka." 

6.  Mica-andesite,  Washoe. '' 

COMPLEMENTARY    NATURE   OF   DACITES   AND    LATET    ANDESITES. 

The  fact  that  the  rhyolite  and  basalt  of  the  district  were  nearly  contemporaneous 
and  probably  complementary,  and  were  perhaps  derived  from  an  original  magma 
like  that  of  the  earlier  andesite,  suggests  that  the  later  andesites  and  dacites,  whose 
eruptions  in  a  general  way  intervened c  between  those  of  the  earlier  andesite  and 
of  the  rhyolite-basalt,  may  also  be  complementary  and  represent  an  earlier  stage 
in  the  differentiation. 

There  is  available  a  single  complete  analysis  of  the  dacite  made  from  a  typical 
specimen  of  the  Brougher  dacite ''  of  Brougher  Mountain  (No.  359).  There  are,  as 
before  stated,  two  complete  analyses  of  the  fresh  later  andesites  (Nos.  225  and  349, 
p.  57).  To  determine  how  far  the  dacite  and  later  andesite  may  be  complementary, 
these  analyses  have  been  added  together  and  halved. 

The  average  of  No.  349,  perhaps  the  freshest  specimen  of  later  andesite,  and  of 
No.  359  (dacite)  is  given  in  column  1  of  the  following  table.  The  average  of  two 
analyses  of  fresh  later  andesite  (Nos.  349  and  225)  was  averaged  with  the  dan  to 
analysis.  The  result  is  given  in  column  2. 

o  Won.  V.  S.  Oeol.  Survey,  vol.  20,  p.  264. 

'•  Ibid.,  p.  282. 

••This  applies  lo  the  Heller  daelte,  the  Fraction  dacite  breccia,  and  the  Tonopah  glassy  rhyolite-dacile.  The  Brougher 
dacite  In  an  exception,  Immediately  succeeding  the  basalt  eruption  of  Mount  Siebert,  and  being  probably  nearly  contem- 
poraneous with  the  Odillc  rhyalite. 

rfSlnce  this  part  of  the  re|x>rt  was  written,  an  analysis  was  made  of  the  glassy  Tonopah  rhyolite-daeite  (No.  661) 
north  of  the  King  Tonopah  shaft,  as  given  on  p.  57.  This  analysis  has  not  been  introduced  into  these  calculations,  sine-i- 
ll offers  no  new  but  only  corroboratory  evidence  concerning  conclusions  here  set  forth.  This  will  appear  from  thu 
following  average  of  the  glassy  Tonopah  rhyollle-dacite  (So.  661)  with  fresh  later  andesite  (No.  849). 

HIO,.  64.28:  AI,O,.  14.98;  Fe,Oa.  3.55;  FeO,  0.71;  MgO,  1.67;  CaO.  3.07:  Na-O.  2.95;  K;O,  4.04. 


COMPLEMENTARY    NATURE    OF   DACITES    AND   ANDESITES.  65 

Mean  composition  of  Tonopah  daciteg  and  later  andesites  compared  with  composition  of  early  andesite. 


i. 

2. 

3. 

4. 

SiOj 

63.98 

64.29 

65.68 

65  13 

A1,O... 

15.09 

15.18 

15.87 

15.73 

Fe,0, 

3.31 

2.72 

1.78 

2.24 

FeO 

.84 

1.05 

1  25 

1  si; 

MgO                

1.60 

1.48 

1.79 

1  49 

CaO 

3.66 

3.90 

3  50 

3  62 

NajO 

3  15 

3  08 

3  20 

2  93 

K20.                   

3.92 

3.76 

3.37 

3  96 

To  compare  these  results  with  known  rocks,  the  nearest  analyses  of  Washoe 
and  Eureka  rocks  are  also  given  above.  No.  3  is  mica-andesite  from  Washoe," 
already  twice  referred  to;  No.  4  is  andesitic  pearlite  from  Eureka.  These  two 
rocks  from  Eureka  and  Washoe  are  among  those  which  are  regarded  (p.  219)  as 
closely  similar  to  the  earlier  andesite  of  Tonopah. 

By  comparison  of  the  different  analyses  it  is  seen  that  the  dacite  and  the 
later  andesite  of  Tonopah  added  together  produce  an  andesite  of  intermediate 
composition,  such  as  is  usually  a  hornblende-andesite  or  a  hornblende-mica- 
andesite.  Moreover,  the  amounts  of  silica,  lime,  soda,  and  potash  in  this  average 
are  strikingly  like  those  in  the  average  of  the  partial  analyses  of  basalt  and 
rhyolite,  as  is  shown  by  the  following  table: 

Analyses  of  siliceous  andesite  compared  with  mean  analysis  of  rhyolite  and  basalt  and  mean  analysis  of  dacite 

and  later  andesite. 


I 

a. 

b. 

Si02 

64  80 

63  98 

64  29 

65  68 

CaO  

3.89 

3  66 

3  90 

4  27 

3  50 

Na,O 

2  79 

3  15 

3  08 

4  08 

3  20 

KjO  

3.51 

3  92 

3  76 

3  17 

3  37 

1.  Average  of  rhyolite  and  basalt  (Nos.  376  and  168,  p.  64). 

2.  Averages  of  later  andesite  and  dacite  (see  table  above). 

3.  Earlier  andesite,  Tonopah  (p.  216). 

4.  Mica-andesite,  Washoe. 

Further  averages  of  the  silica,  lime,  soda,  and  potash  of  the  dacite  and  later 
andesite  may  be  had  by  combining  with  the  andesite  analyses  the  partial  analyses 
of  the  dacite  (No.  368)  from  the  east  end  of  Butler  Mountain  and  of  the  dacite 


"Mon.  U.  S.  Geol.  Survey,  vol.  20,  p.  282. 


16843— No.  42—05- 


66  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 

(No.  388)  from  the  south  side  of  Golden  Mountain.     If  each  of  these  is  combined 
separately  with  the  later  andesite  analysis,  No.  349,  the  result  is  as  follows: 

Comparison  of  mean  analyses  of  daeites  and  andesites. 


1  (349  and 
368). 

2  (349  and 
388). 

3. 

SiO,                                   

64.63 

65.91 

65.15 

CaO                                                                     ..                

3.31 

3  11 

3  60 

NajO  

3.37 

3.62 

3.29 

KZO                       

4.07 

3.96 

3.83 

1,  2.  Averages  of  later  andesite  and  dacite. 

3.  The  average  of  the  fresh  later  andesite  specimens  349  and  225  is  averaged  with  the  average 
of  the  three  dacite  analyses,  359,  368,  and  388. 

STATEMENT  OF  DIFFERENTIATION  THEORY. 

These  considerations  suggest  that  an  original  magma  of  composition  similar 
to  that  of  the  earlier  andesite  has  split  up  by  differentiation,  first  into  a  more 
basic  andesite  (later  andesite)  and  a  siliceous  dacite,  and  later,  by  continuation  of 
the  process,  into  a  siliceous  rhyolite  and  a  basalt,  as  follows:" 

Intermediate  andesite. 
Basic  andesite.  Siliceous  dacite. 

Basalt.  Siliceous  rhyolite. 

SUMMARY  OF  GEOLOGICAL,  HISTORY. 

Previous  to  the  Tertiary  period,  Paleozoic  limestone,  intruded  by  granitic 
rocks,  occupied  this  region.  With  the  Tertiary  began  a  period  of  volcanism, 
attended  by  the  accumulation  of  lake  sediments  and  subaerial  deposits  in  inclosed 
basins.  These  deposits  began  in  the  Eocene,  and  beds  belonging  to  this  epoch 
are  found  near  Tonopah,  though  not  within  the  area  mapped. 

About  8  miles  north  of  Tonopah  and  1  mile  west  of  the  little  mining  camp 
of  Ray  the  writer  found  a  series  of  folded  gravels,  tuffs,  lavas,  and  some  white, 
thin  limestones  carrying  numerous  Eocene  fossils.  These  were  sent  to  Dr. 
W.  H.  Dall  for  determination,  who  remarks: 

"According  to  the  literature  the  fresh-water  beds  from  which  these  fossils 
came  have  been  referred  by  Doctor  White  and  Meek  to  the  Wasatch,  or  Bear 
River  Laramie,  Eocene,  which  is  believed  to  be  nearly  the  equivalent  of  the 
lower  Eocene  or  Chickasawan  marine  Eocene  (Lignitic  of  old  authors)  of  our 
southeastern  coastal  plain.  The  species  are: 

"This  corresponds  with  the  scheme  for  the  general  succession  of  IMVMS  In  the  Orcat  Basin,  as  outlined  by  the 
writer  (Jour.  Qeol.,  vol.  8,  p.  643),  and  reaches  the  same  conclusion  that  is  already  arrived  at  from  independent 
considerations.  It  coincides,  as  the  writer  has  previously  pointed  out,  with  the  luw  previously  deduced  by  Iddings 
from  ntudy  of  the  volcanic*  of  the  Oreat  Basin  and  other  regions  (Bull.  Phil.  Soc.  Wash.,  vol.  12,  p.  145). 


SUMMARY    OF    GEOLOGICAL    HISTORY.  67 

"  Vivipara,  close  to  if  not  V.  couesi;  Planorbis  utahensis  Meek;  Ancylus  3  sp. ; 
and  a  small  bivalve,  probably  a  Corbicula,  but  which  I  suspect  to  be  the  same  as 
Sphserium  idahoense  Meek.  The  specimens  are  merely  internal  casts,  but  if  they 
are  really  Corbicula  may  prove  to  be  C.  occidetitalis  Meek.  Their  condition  is 
too  imperfect  to  be  certain  even  of  the  genus,  but  the  form  closely  approaches 
that  of  the  figures  of  S.  idahoense.'''' 

These  overlie  the  Paleozoic  limestones  near  Ray.  Similar  beds  were  noted 
at  several  places  between  Ray  and  Sodaville.  They  are  probably  continuous  with 
a  part  of  the  Tertiary  deposits  of  the  Silver  Peak  and  Monte  Cristo  mountains." 

The  oldest  of  the  Tertiary  rocks  within  the  area  of  the  Tonopah  map  are 
probably  early  Miocene,  and  so  far  as  known  the  volcanic  manifestation  began 
with  an  eruption  of  andesite.  In  this  andesite  were  formed  fracture  zones,  along 
which  heated  waters  ascended  and  deposited  the  valuable  veins  of  the  region. 
Another  extensive  eruption  of  similar  but  slightly  more  basic  andesite  followed, 
and  then  there  was  probably  a  period  of  volcanic  rest  and  of  denudation. 
Eruption  was  resumed  by  the  outbreak  of  volcanoes,  which  alternate!}-  ejected 
siliceous  dacite  and  poured  out  volcanic  mud  and  frequently  pumiceous  lava.  Some 
of  the  material  may  have  been  accumulated  in  water;  most  of  it  was  probably 
deposited  upon  the  land.  Later,  more  glassy  dacite  of  a  slightly  different 
composition  ascended  from  below  in  irregular  channels  and  poured  out  on  the 
surface  as  thin  sheets,  or  exploded  and  formed  tuffs.  Heated  ascending  waters 
followed  the  intrusive  contacts  of  this  lava  and  formed  a  group  of  quartz  veins 
which  contain  gold  and  silver,  but  which  are  less  important  as  regards  strength 
and  values  than  the  veins  formed  after  the  eruption  of  the  earlier  andesite. 

As  these  dacite-rhyolite  eruptions  quieted  down  a  lake  was  formed  in  a  basin, 
which  may  have  been  due  to  a  depression  of  the  crust  consequent  upon  the 
previous  copious  eruptions.  In  this  lake  there  accumulated  quietly  several 
hundred  feet  of  sediments,  with  occasional  light  showers  of  ash  from  volcanoes, 
and,  in  the  lower  portions,  some  thin  flows  of  dacite  lava.  Then  the  beds  were 
lifted  and  became  dry  land.  This  uplift  may  have  been  due  to  the  accumulation 
of  additional  volcanic  material  beneath  this  portion  of  the  crust.  Streams  began 
to  cut  into  the  lake  beds,  the  uplift  was  continued,  and  the  whole  district  was 
tilted  bodily  to  the  west  at  an  average  angle  of  20°.  After  this  there  were 
renewed  outbursts,  from  probably  new  vents,  which  doubtless,  corresponded,  in 
part  at  least,  to  the  present  mountains.  On  Brougher  and  Butler  mountains 
explosive  eruptions  occurred,  the  material  being  dacitic,  like  that  immediatelv 
preceding  the  lake  deposits.  Cones  of  ash,  cinders,  and  bombs  were  built  up, 
and  there  were  occasional  very  thin  and  scant}'  glassy  flows.  On  Siebert 

a  Turner,  H.  W.,  Twenty-first   Ann.  Kept.  U.  S.  Geol.  Survey,  pt.  2,  pp.  192-244;   Spurr,  J.  E.,  Bull.  U.  S.  Geol. 
Survey  No.  208,  PI.  1,  and  pp.  105-106, 185. 


68  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,  NEVADA. 

Mountain  there  was  an  explosive  outburst  of  basaltic  material,  followed  by  a 
thin  basalt  flow.  Subsequently  columns  of  liquid  lava  welled  up  and  stood  in 
the  vents  of  the  volcanoes,  but  did  not  outflow.  Some  of  these  were  composed  of 
dacite,  some  of  rhyolite.  As  these  columns  cooled,  heated  waters  rose  along 
their  contacts  and  deposited  chalcedony  and  other  minerals,  and  mud  dikes  were 
injected  into  the  soft  intruded  rocks.  The  explosive  outbreaks  and  the  intrusion 
of  these  large  necks  must  have  broken  the  rocks  into  blocks  and  displaced  the 
blocks,  for  at  this  time  many  faults  were  formed. 

On  the  cessation  of  this  dacite- rhyolite  period  of  volcanic  activity  there  was 
a  collapse  or  depression  around  the  vents.  This  sinking  took  place  largely  along 
the  fault  planes,  and  was  especially  prominent  around  the  volcanic  necks,  which 
as  they  sagged  dragged  down  blocks  of  the  intruded  older  rocks  with  them. 

Since  this  time,  which  was  probably  somewhere  in  the  Pliocene,  erosion  has 
been  active,  stripping  away  the  debris  covering  from  the  dacite-rhyolite  necks, 
and  leaving  them  as  hills,  and  in  general  removing  the  surface  layers  from  the 
hills  to  the  desert  valleys. 

AGE  OF  THE  ROCKS  AT  TONOPAH. 

It  is  known  that  all  these  volcanic  rocks  are  of  Tertiary  age.  They  belong 
to  a  series  of  lavas  which  occupy  a  large  part  of  the  Great  Basin  and  whose 
Tertiary  age  has  been  established. 

Place  of  Tonopah  lavas  in  Great  Basin  volcanic  history. — Some  years  ago" 
the  writer  attempted  to  classify  the  known  facts  concerning  the  nature  and 
succession  of  the  lavas  in  this  region.  He  found  that  in  many  places  the  same 
lavas  occur  in  much  the  same  relative  quantity,  have  nearly  the  same  mineralogical 
composition,  and  give  evidence  of  about  the  same  relative  age.  Moreover,  where 
two  or  more  of  these  lavas  are  found  close  together,  their  order  of  succession  is 
in  general  much  the  same,  although  at  any  given  place  certain  members  of  the 
series  may  be  lacking.  In  no  one  locality  has  the  complete  succession,  as 
indicated  by  the  correlation  of  all  the  sections,  been  observed;  but  in  order  to 
find  it,  gaps  in  one  place  may  be  filled  from  observations  in  another. 

The  result  of  this  comparison  was  the  separation  of  the  Tertiary  lavas  into 
five  successively  erupted  groups,  as  follows: 

1.  Rhyolites. 

2.  Hornblende-biotite-pyroxene-andesites,  followed  by  dacites. 

3.  Rhyolites,  Homi-tinies  accompanied  by  basalts. 

4.  Pyroxene-andantes. 

5.  Basalts,  sometimes  acc'omi>amed  by  rliyolites. 


aSuccemlon  and  relation  of  lava«  In  the  Ureat  Basin  region:  Jour.  Geol.,  vol.  8.  No.  7.  pp.  621-646. 


AGES    OF    THE    BOCKS.  69 

At  Tonopah  the  succession  of  lavas,  as  above  worked  out,  may  be  expressed 
as  follows: 

(a)  Hornblende-biotite-andeaite. 
Biotite-augite-andesite. 

(b)  Dacites  and  rhyolites,  with  a  little  basalt. 

These  may  be  assumed  to  coincide  with  2  and  3  of  the  above  general  grouping. 

Probable  Neocene  age. — In  the  comparative  study  above  referred  to"  available 
data  were  accumulated  for  determining  roughly  the  age  of  the  different  groups 
with  reference  to  the  standard  divisions  of  geologic  time  and  to  the  different 
periods  of  Tertiary  lakes  as  defined  by  King  in  his  .-ummary  of  the  results  of 
the  Fortieth  Parallel  Survey.  The  eruption  of  group  No.  2  (the  hornblende- 
biotite-pyroxene-andesites,  followed  by  the  dacites)  occurred  between  the  end  of 
the  Eocene  and  the  latter  part  of  the  Miocene,  and  was  contemporaneous  with 
the  Miocene  lakes,  while  that  of  No.  3  (rhyolites,  sometimes  accompanied  by 
basalts)  extended  from  the  latter  part  of  the  Miocene  well  into  the  Pliocene,  to 
the  time  of  the  beginning  of  the  Pliocene  Shoshone  Lake.  On  the  assumption 
that  the  correlation  of  the  Tonopah  lavas  above  given  is  correct,  the  andesites, 
both  earlier  and  later,  would  belong  to  the  first  half  of  the  Miocene  and  to  the 
Miocene  lake  period;  while  the  dacites,  rhyolites,  and  basalts  would  extend  from 
near  the  middle  of  the  Miocene  into  the  Pliocene,  and  would  be  partlv 
contemporaneous  with  the  latter  part  of  the  Miocene  lake. 

INFUSORIA    IN    THE    SIEBERT    TUFFS. 

In  the  white  tuffs  at  the  east  base  of  Siebert  Mountain  a  stratum,  not 
distinguished  in  the  field  from  the  more  ordinary  white  rhyolitic  or  dacitic  tuff, 
was  shown  by  the  microscope  to  be  entirely  made  up  of  minute  diatoms  or  infu- 
soria. These  were  recognized  by  the  writer  as  probably  similar  to  species  described 
by  Mr.  King  as  occurring  in  the  deposits  of  the  Miocene  lakes  of  Nevada.  At 
the  time  the  recognized  succession  of  lavas  did  not  seem  compatible  with  this 
idea,  and  the  thin  section  was  referred  to  Dr.  Rufus  M.  Bagg,  jr.,  for  examina- 
tion. Subsequently,  it  is  proper  to  add,  new  discoveries  as  to  the  lava  succession 
removed  the  difficulties  in  the  way  of  considering  the  deposits  Miocene. 

Doctor  Bagg's  report  follows: 

"The  material  submitted  me  from  Tonopah,  Nev.,  for  examination  consists  of 
innumerable  diatoms  which  belong  almost  exclusively  to  two  species,  Helosira 
granulala,  L.  W.  Bailey,  and  Melosira  varians,  Ag.,  the  latter  being  considerably 
less  abundant  than  the  former. 

"Jour.  Geol.,  vol.  8,  No.  7,  p.  637. 


70  GEOLOGY   OF   TONOPAH   MINING    DISTRICT,   NEVADA. 

"This  species,  Melosira  granulata,  is  synonymous  with  Ehrenberg's  Gallionella 
granulata,  and  other  synonyms  for  the  species  are  Melosira  punctata,  Gallionella 
marchica,  G.  procera,  and  G.  tenerrima. 

"I  can  discover  no  species  in  the  material  sent  me  which  would  limit  the  deposit 
to  the  Miocene  age,  for  the  most  abundant  form,  M.  granulata,  is  living  to-day  in 
the  Para  River,  South  America,  and  elsewhere,  as  well  as  occurring  fossil  in 
Tertiary  deposits. 

"There  is  nothing  to  prevent  this  deposit  from  being  regarded  as  Pliocene  if 
stratigraphical  evidence  warrants  this  view,  but  the  deposit  was  laid  dow-n  in  fresh 
water.  In  addition  to  the  two  species  above  given  there  are  a  few  forms  of  Coscino- 
dlscus  radiatm." 

COMPARISON  OK   SIEBERT    TUFFS  WITH    MIOCENE    PAH-UTE    LAKE    DEPOSITS. 

Miocene  deposits  have  been  described  b3'  King  in  western  Nevada"  between 
the  one  hundred  and  seventeenth  meridian  and  the  Sierra  Nevada.  These  deposits 
are  always  upturned,  dipping  from  10°  to  25°,  and  they  are  frequently  cut  through 
and  overflowed  by  basalt.  They  are  usually  made  up  of  volcanic  materials,  and 
are  several  thousand  feet  thick.  They  contain  beds  of  white  and  yellow  infusorial 
silica,  and  on  the  northeast  point  of  the  Kawsoh  Mountains,  where  the  strata  are 
tilted,  eroded,  and  covered  by  caps  of  basaltic  rock  (as  on  Siebert  Mountain  in 
Tonapah),  the  following  species  were  most  abundant: 

Gallionella  granulata. 
Gallionella  sculpta. 
Spongolithis  acicularis. 

These  also  were  recognized: 

Pinnubaria  ilia-quails,  and 
Coscinodiscus  radiatus. 

The  age  of  these  beds  is  determined  more  especially  by  molluscan  and 
mammalian  fossils,  found  elsewhere. 

These  beds,  therefore,  are  of  the  same  character  as  the  Siebert  tuff  at 
Tonopah,  which  was  deposited  in  the  rhyolite-dacite  period,  and  suggest  that 
the  lake  in  which  the  tuffs  were  deposited  is  identical  with  the  Miocene  Pah-Ute 
Lake  of  King.6  The  tilting  and  amount  of  erosion  of  the  Tonopiih  white  tuffs 
prevents  any  correlation  with  the  Pliocene  lake  (Lake  Shoshone)''  beds,  whose 
distribution  frequently  bears  a  close  relation  to  the  present  topographic  basins, 
and  which  are  little  disturbed. 


a  U.S.  Geol.  Expl.  Fortieth  Har.,  vol.  1,  p.  412etseq.  l> Op.  cit.,  p.  454.  cOp.  cit..  p.  466. 


AGES    OF    THE    VOLCANIC    BOCKS. 
CONCLUSION. 


71 


It  may  be  provisionally  concluded  that  the  volcanic  rocks  at  Tonopah,  from 
the  earlier  andesites  to  the  Brougher  dacites  and  the  rhyolites,  were  erupted 
between  the  early  Miocene  and  some  time  in  the  first  half  of  the  Pliocene. 

The  following,  then,  is  the  sequence  of  events  as  deciphered  for  the  vicinity 
of  Tonopah  (fig.  10): 


Hypothet-  Hypothet-   Earlier         Later          Dacite 
ical  deep-    ical  deep-   andeatte.    andesite.       breccia, 
seated          seated 
granite,     limestone. 


Tonopah    Lake  beds.     Faults, 
rhyollte- 
daelte. 


Later  da-  Earlier  ande- 
cite  and       site  veins 
rhyolite    {lesser  veins 
intrii-      belonging  to 
slons.      other  periods 
not  repre- 
sented). 


FIG.  10.— Ideal  cross  section  of  Tonopah  rocks.    (This  section  does  not  represent  any  particular  place,  and  is  simply 
intended  to  illustrate  the  geologic  conditionsa-s  described  in  the  text.) 

Sequence  of  formations  and  erent-s  in  the  vicinity  of  Tonopah. 
Earlier  andesite. 
Fracturing. 

Vein    formation.      Primary    minerals,    quartz,     adularia     (valencianite),    carbonates    of    lime, 
magnesium,  and  manganese,  stephanite,  polybasite,  argentite,  silver  selenide,  galena,  pyrite, 
chalcopyrite,  etc.     Values  good;  gold  and  silver,  silver  predominant. 
Erosion. 
Later  andesite. 
Probable  erosion. 
Heller  dacite. 
Fraction  dacite  breccia. 

Tonopah,    rhyolite-dacite    breccias,    flows,    and    dikes,    intermingled    with     slightly    stratified     or 
unstratified  pumiceous  or  tuffaceous  fragmental  material. 

Vein  formation.      Primary   minerals,    quartz,    pyrite,    barite.      Values    usually   relatively  low; 
gold  and  silver,  gold  apt  to  predominate. 


72  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 

Erosion. 

Siebert  tuffs  (lake  beds)  deposited,  with  an  occasional  thin  dacite  flow. 

Elevation  of  tuffs. 

Tilting. 

Basalt. 

Chief  faulting.     Affects  eyerything  preceding. 

Rhyolite  intrusion  (Ararat,  Oddie,  Rushton  hills). 

Vein  formation.     Primary  minerals,  quartz,  chalcedony,  calcite,  siderite,  pyrite,  etc.     Values 

low;  gold  and  silver,  gold  apt  to  predominate. 
Brougher  dacite  intrusion  (Butler,  Brougher,  Golden,  Siebert  mountains). 

Mineralization  (chalcedony,  manganese).     Values  slight  to  insignificant.     Mud  veins. 
Erosion. 

Latest  rhyolite-dacite  flow   (slopes  of  Oddie  and  Brougher). 
Erosion. 

PRINCIPLES   OF  FAULTING. 

The  chief  recognized  faulting  of  the  district  has  already  been  described  (p.  47) 
as  attendant  and  consequent  upon  the  Brougher  dacite  intrusion.  The  writer  deems 
it  unnecessary  to  attempt  to  describe  separately  the  evidence  and  effect  of  each 
fault.  Their  locations  and  the  general  nature  of  their  displacement  are  shown  on 
the  areal  geology  map.  Their  underground  courses  and  intersections  are  doubtless 
complicated,  and  their  study  would  constitute  a  geometrical  problem  in  three 
dimensions  for  the  solution  of  which  there  are  in  most  cases  no  sufficient  data.  On 
account  of  the  irregular  thickness  and  extent  of  each  of  the  volcanic  formations 
at  Tonopah,  projection  far  beyond  actual  observation  can  not  safely  be  made;  so 
no  general  cross  sections  have  been  constructed. 

Valuable  observations  on  faulting  have  been  made  underground,  however,  in 
some  of  the  mines,  especially  where  veins  have  afforded  measures  of  displacement. 
It  has  been  found  impracticable  to  separate  the  account  of  such  faulting  from  the 
discussion  of  the  veins  which  they  affect,  so  the  reader  is  referred  to  such 
discussions,  particularly  to  those  concerning  the  Fraction,  Wandering  Boy,  Valley 
View,  Mizpah,  and  Montana  Tonopah  workings  (pp.  115-176). 

CRITERIA  OF  FAULTING. 

It  is  worth  while  to  record  the  manner  in  which  the  structure  has  been 
worked  out  in  this  complicated  region.  Although  the  region  mapped  embraces 
only  about  6  square  miles,  and  outcrops  are  very  nearly  continuous,  several  months 
of  study  were  necessary  to  reach  an  approximately  satisfactory  solution  of  the 
areal  geology.  Ideas  concerning  the  structure  were  successively  exchanged  for 
newer  ones  as  fact  after  fact  was  brought  to  light.  The  existence  of  faulting  was 
strongly  suspected,  from  topographic;  evidence,  from  the  time  of  arrival  in  the  field, 


FAULTING.  73 

but  the  final  results  proved  that  in  every  case  the  faults  assumed  from  such 
evidence  were  not  faults,  while  the  ultimate  discovery  of  numerous  and  important 
faults  was  due  to  careful  study  of  the  rocks. 

When  by  close  examination  and  correlation  of  facts  the  complicated  and  often 
closely  related  rocks  were  satisfactorily  separated  into  stratigraphic  units,  after 
numerous  unsatisfactory  attempts,  the  most  important  step  toward  the  elucidation 
of  the  geologic  history  and  structure  had  been  taken.  But  still  the  most  extreme 
caution  was  necessary,  for  while  the  local  geologic  column  was  probably  historically 
correct  for  the  whole  district,  there  were  many  local  gaps  and  irregularities.  As 
there  were  several  periods  of  apparently  active  but  irregular  erosion  between 
volcanic  outbursts  and  as  the  distribution  of  many  of  the  members  of  the  series 
was  limited  and  irregular  it  seemed  that  any  member  might  rest  directly  upon  &ny 
older  one,  the  intervening  ones  being  unrepresented,  while  a  few  hundred  yards 
away  the  represented  succession  would  be  different.  For  similar  reasons  it  was  not 
possible  to  reckon  upon  any  constant  thickness  for  any  formation;  in  one  place  it- 
might  be  a  few  feet  thick,  in  others  hundreds.  So  the  ordinary  stratigraphic 
criteria  of  faulting  were  very  inconclusive. 

SIEBERT   TUFF   BOUNDARIES. 

The  key  to  the  problem,  undoubtedly  was  the  determination  of  the  geologic 
position  of  the  Siebert  tuff,  which  consists  of  characteristic  finely  stratified  thick 
beds.  In  working  out  the  structure  the  first  thing  done  was  to  carefully  follow 
the  limits  of  these  Siebert  tuff  areas.  It  was  found  that  in  most  cases  these  were 
separate;  they  reappear  in  different  parts  of  the  area  mapped  and  are  bounded  on 
several  sides  by  straight  lines.  This  fact  immediately  suggested  the  existence  of 
numerous  intersecting  faults. 

Where  a  rectilinear  boundary  of  a  Siebert  tuff  area  ran  transversely  to  the 
strike  of  the  beds,  a  fault  was  evident,  in  case  the  contiguous  rock  was  not 
intrusive.  In  the  case  of  a  surface  formation,  like  the  Fraction  dacite  breccia, 
this  evidence  was  conclusive,  and  parts  of  the  majority  of  detected  faults  were 
followed  in  this  way.  Similarly,  if  a  fault  was  parallel  with  the  strike,  and  the 
dip  of  the  tuff  would  carry  it  below  a  contiguous  rock  (as  the  Fraction  dacite 
breccia,  for  example)  which  was  known  to  be  lower  in  the  geologic  column  than 
the  tuff,  the  nature  of  the  contact,  as  due  to  dislocation,  was  evident. 

DIKES  ALONG   FAULT   ZONES. 

Another  criterion,  perhaps  not  so  important,  was  developed  by  the  discovery 
that  the  Brougher  dacite  sent  out  dikes  along  some  of  the  faults,  as  along  the  Cali- 
fornia fault.  (See  map,  PI.  XVI.)  This  showed  at  once  that  the  dacite  reached  its 
present  position  essentially  subsequent  to  the  faulting  (a  conclusion  which  was  other- 


74  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 

wise  abundantly  verified),  and  that  the  dikes  running  out  from  the  volcanic  centers 
occupied  at  times  fault  zones.  These  dikes  were  then  traced,  and  when  they  were 
conspicuously  straight  and  narrow  their  course  was  critically  examined  to  deter- 
mine whether  it  could  possibly  be  a  fault  plane.  Often  such  dikes  are  intermit- 
tent, appearing  only  in  small  outcrops  here  and  there  along  the  line,  with  no 
visible  connection.  Such  a  condition  was  still  more  strongly  suggestive  of  a 
fracture  zone.  Frequently  the  examination  of  the  rocks  on  both  sides  of  such  a 
line  confirmed  the  suspicion  of  faulting,  and  important  faults  were  discovered  in 
this  way. 

BOUNDARIES   OF  LAVAS. 

A.S  the  knowledge  of  the  different  formations  increased  it  became  possible  to 
draw  their  boundaries  with  frequently  great  accuracy.  Where  these  were  recti- 
linear, as  in  the  case  of  the  tuft's,  and  could  not  reasonably  be  interpreted  as 
normal  contacts  caused  by  the  outcropping  of  inclined  formations,  and  one  for- 
mation could  not  have  been  intruded  into  the  other,  faults  were  considered  to  be 
indicated.  Even  in  the  case  of  two  volcanic  rocks,  like  the  earlier  and  later 
andesite  on  Mizpah  Hill,  the  boundaries,  though  obscure  and  traceable  with 
difficulty  on  the  surface,  could  finally  be  determined  to  be  rectilinear,  intersecting, 
and  probably  due  to  faulting.  In  this  case  the  veins  afforded  valuable  evidence, 
for  their  outcrops  were  cut  clean  off  along  the  fault  planes. 

EROSION    FAULT   SCARPS. 

As  the  perception  of  the  real  connection  between  the  strati  graph}'  and  structure 
and  the  topography  grew,  the  latter  often  became  an  efficient  guide.  The  underlying 
rocks  have  exercised  a  remarkably  efficient  control  over  the  surface  forms.  Where 
two  rocks  of  unequal  hardness  are  brought  together  by  faults,  the  harder  rock  will 
rise  above  the  softer  in  a  more  or  less  perceptible  scarp.  With  the  exception  of  the 
rhvolites  and  the  Brougher  dacite,  and  to  a  less  degree  of  the  silicified  earlier 
andesite,  however,  the  difference  in  resistance  of  the  rocks  is  not  great.  The 
Fraction  dacite  breccia  and  the  glassy  Tonopah  rhyolite-dacite  in  the  southern  part 
of  the  area  mapped  are  chiefly  friable  fragmental  surface  deposits,  while  the  later 
andesite  disintegrates  rapidly.  The  Siebert  tuff  is  softer  than  the  others,  and 
when  sufficiently  removed  from  the  influence  of  a  protecting  harder  rock,  forms 
flat,  smooth  areas,  on  whose  boundaries  fault  contacts  are  apt  to  be  marked  by 
slight  but  pronounced  scarps,  usually  only  a  few  feet  high,  since  the  adjacent 
rock  is  apt  to  be  very  little  harder.  These  slight  scarps  afford  strong  preliminary 
evidence,  and  invite  the  closest  searching  after  stratigraphic  corroboration. 

Nearly  every  topographic  feature  in  the  Tonopah  district,  however  small,  is 
due  to  the  nature  of  the  underlying  rock;  thus  many  straight  depressions  or 
slight  valleys  are  probably  due  to  the  easier  erosion  of  a  fractured  or  faulted 


FAULTING.  75 

zone,  as  compared  with  the  less  fractured  rock  on  each  side.  Such  is  probably 
the  case  with  the  northeast  depression  at  the  southeast  base  of  Brougher  Mountain, 
and  with  other  creases  in  the  surface. 

SCARP  PHENOMENA  WEST  OF  BROUGHER  MOUNTAIN. 

Some  especially  interesting  observations  on  the .  surface  configuration  as  an 
indication  of  faulting  were  made  in  the  comparatively  flat  area  in  the  west  part 
of  the  district  mapped,  west  and  northwest  of  Brougher  and  Siebert  mountains, 
respectively.  Here  rhyolitic-dacitic  breccias,  chiefly  detrital,  are  intermingled  with 
tuft's,  so  that  they  sometimes  can  be  distinguished  only  with  difficulty  from  the 
main  overlying  Siebert  tuff.  Where  the  Siebert  tuff  is  certainly  distinguishable 
the  rectilinear  intersecting  boundaries  show  that  complicated  faulting  has  taken 
place,  but  the  mass  of  rhyolite-dacite  breccias  offered  at  first  little  suggestion  as 
to  structural  relations. 

When  this  area  is  viewed  from  an  eminence,  as  from  Brougher  Mountain  or 
from  the  hill  west  of  Siebert  Mountain,  just  beyond  the  area  mapped,  there  is 
seen  a  significant  series  of  parallel  ridges  which  were  at  once  surmised  to  indicate 
the  presence  of  faults.  From  the  hill  last  referred  to,  these  slight  scarps  are  seen 
to  bound  areas  which  have  rectilinear  outlines,  and  which  are  plainly  distinguished 
in  tint  from  one  another,  one  being  purplish,  another  reddish,  and  so  on.  A  minute 
study  strengthened  the  conjecture  that  in  this  region  there  are  complicated  and 
numerous  intersecting  faults.  It  was  concluded  that  these  faults  brought  into 
juxtaposition  the  Tonopah  rhyolite-dacite  breccia,  the  Fraction  dacite  breccia,  the 
Siebert  tuff,  or  different  parts  of  any  one  of  these,  and  that  the  resulting  erosion 
brought  out  the  harder  blocks,  which  were  thus  bounded  by  straight  scarps,  usually 
of  slight  relief.  The  Tonopah  rhyolite-dacite  breccia,  being  harder,  nearly  always 
occupies  the  relatively  elevated  portions,  while  the  soft,  Fraction  dacitic  breccia  and 
the  Siebert  tuff  lie  in  the  depressions.  These  depressions  are  covered  with  a  slight 
thickness  of  detritus,  but  prospect  holes  show  in  almost  every  case  that  they  are 
floored  with  the  softer  breccias.  The  straight  boundary  lines  are  strongly  con- 
trasted with  the  irregular  unfaulted  contact  of  the  glassy  Tonopah  rhyolite-dacite 
in  the  north  corner  of  the  area  mapped. 

DESCRIPTION   OF   ZIGZAG   SCARPS. 

One  or  two  of  the  most  interesting  occurrences  of  these  slight  scarps  were 
made  the  subjects  of  especial  study.  Between  Siebert  and  Brougher  mountains 
the  flat  area  floored  by  the  dacitic  breccias  and  by  the  Siebert  tuffs  reveals  to 
the  close  observer  certain  straight  lines,  which  are  apparently  slight  ridges  and 
depressions  in  the  detritus,  but  which  are  really  closely  underlain  by  the  soft 
bed  rocks,  though  these  outcrop  only  occasionally.  In  this  area  the  occurrence 
of  a  number  of  faults  was  proved  by  stratigraphic  evidence,  chiefly  by  the 


76 


GEOLOGY    OK   TONOPAH   MINING    DISTRICT,   NEVADA. 


rectilinear  boundaries  of  the  Siebert  tuffs.  The  position  of  one  such  fault,  marked 
A  on  the  accompanying  diagram  (PI.  XII),  was  determined  by  stratigraphic 
evidence  for  a  part  of  its  course,  as  will  be  noted  by  consulting  the  geologic 
map  (PI.  XI).  Eastward  of  this  part,  however,  it  is  bordered  apparently  on  both 
sides  by  the  tuff,  yet  along  the  continuation  of  the  line  established  by  strati- 
graphic  evidence  there  is  on  its  north  side  a  slight  scarp  about  10  feet  high. 
Just  north  of  this  scarp  a  similar  scarp,  of  about  the  same  height,  and,  like 
the  former  one,  facing  to  the  south,  runs  in  a  straight  line,  but  in  a  direction 
more  nearly  east  and  west  than  the  one  first  mentioned.  Toward  the  east  the 
foot  of  this  scarp  is  in  the  bottom  of  a  narrow  depression;  toward  the  west, 
where  the  depression  broadens,  the  scarp  lies  on  the  north  side.  In  this  broader 
portion,  however,  the  other  side  of  the  depression  has  little  or  no  scarp,  is  at  a 
maximum  of  3  or  4  feet  in  height,  varying  from  that  to  nothing,  and  has  no 
straight  or  rectilinear  course  (fig.  11).  This  first-mentioned  scarp  is  continued 


FIG.  11.— Cross  section  of  water  runway,  usually  dry  (c-d  of  PI.  XII),  showing  bold,  straight  scarp  on  left,  believed  to  be 
consequent  on  faulting,  and  low,  curved  bank  on  right,  believed  to  be  due  to  occasional  drainage. 

farther  west,  but  is  set  off  en  echelon,  although  the  corners  are  slightly  rounded; 
the  set-offs  are  always  in  a  northerly  direction  and  the  main  trend  corresponds  to 
that  of  the  straight  scarp  farther  east.  With  a  slight  interruption,  caused  by  the 
incoming  of  a  depression  which  is  probably  due  to  an  unusually  soft  fault  block, 
this  scarp  continues  northwestward  beyond  the  area  mapped,  and  can  be  followed 
with  the  eye  a  considerable  distance  farther,  toward  the  little  eminence  called 
Table  Mountain.  A  sighted  line  along  the  scarp  near  the  western  limit  of  the 
map  has  a  general  direction  of  N.  65°  W.  On  examination,  however,  the  front 
of  the  scarp,  which  has  a  uniform  height  of  10  or  15  feet,  and  which  always 
faces  the  south,  is  found  to  be  continuously  set  off  en  Echelon  in  the  same  sense 
and  fashion  as  the  portion  farther  east.  The  conditions  are  indicated  in  PI.  XII. 
The  two  chief  alternating  directions  of  the  scarp  faces  are,  (1)  chief,  N.  85°  E., 
(2)  minor  (set-offs),  N.  45°  W.  Along  the  whole  of  its  course  the  relative  depres- 
sion to  the  south  of  the  scarp  is  used  as  a  runway  for  the  occasional  surface 
waters,  and  can  easily  be  mistaken  for  a  depression  due  simply  to  erosion. 
However,  the  south  side  of  this  depression  does  not  partake  at  any  point  of  the 
peculiarities  of  the  north  side,  being  low  and  irregular  in  course,  and  without 


US   GEOLOGICAL   SURVEY 


PROFESSIONAL    PAPER    NO    42     PL    XII 


DIAGRAMMATIC  MAP  SHOWING  TWO  PARALLEL  ZIGZAG  SOUTH-FACING  SCARPS 

THE  SOUTHERN  ONE  ABOUT 1O  FEET  HIOH.THE  NORTHERN  ONE  25  FEET  HIGH 

By,  I.E.  Spun- 
Scale 

IQOO 5OQ  O  1OOO  2OOO  rjOOOfVet 

Contourmtei-val2O  feet 


FAULTING.  77 

any  definite  continuous  scarp.  Moreover,  the  jogs  in  the  scarp  under  considera- 
tion can  not  he  explained  by  stream  erosion,  for  they  are  not  at  the  entrance  of 
auxiliary  gullies,  the  angle  of  the  jog  forming  practically  an  unbroken  wall. 

ZIGZAG    SCARPS   EXPLAINED   BY    FAULTING. 

The  phenomenon  described  can  hardly  be  explained  except  as  controlled  by 
faulting,  and  two  intersecting  systems  are  indicated.  Corroborative  evidence  of 
this  conclusion  is  present.  Along  the  western  portion  of  the  scarp  where 
examined  there  occur  at  different  points  isolated  outcrops  of  light-colored  dike 
rhyolite  that  has  the  characteristics  of  the  Oddie  rhyolite,  and  is  distinct  from 
the  glassy  Tonopah  rhyolite-dacite  with  which  it  is  in  contact.  These  dikes  are 
intermittent  rather  than  continuous,  but  form  distinct  jogs  parallel  with  the  set- 
offs  of  the  scarp.  It  is  known  that  this  rhyolite  sometimes  forms  dikes  along 
faults  in  this  district  and  is  later  than  the  main  faulting. 

CONSEQUENCES   OF   EXPLANATION. 

The  chief  or  longer  scarp  faces  are  parallel  to  the  straight  scarp  into  which 
the  jogged  scarp  runs  farther  east  (j?,  in  PI.  XII),  while  the  shorter  or  minor 
faces  are  parallel  with  the  slight  scarps  lying  a  short  distance  farther  north, 
limiting  probable  fault  blocks,  as  already  described.  It  appears,  then,  that  the 
jogged  scarp  is  the  result  of  two  sets  of  intersecting  faults,  and  from  the  figure 
it  is  evident  that  when  the  dimensions  of  the  jogs  are  diminished  the  course 
of  the  resultant  will  approach  a  straight  line,  and  indeed  may  do  so  to  such 
a  degree  as  to  be  practically  indistinguishable  from  such  a  line.  By  the  pre- 
dominance of  one  set  of  faulting  over  the  other  set  the  resultant  line  may  lie  in 
any  given  direction  and  may  be  straight  or  curved.  The  line  made  by  joining 
the  points  of  the  sharp  spurs  along  the  scarp,  indicating  the  general  resultant 
of  the  two  systems  of  jogs,  is  parallel  with  the  scarp  first  mentioned,  which 
lies  farther  east  (A,  in  PI.  XII).  It  is  possible,  therefore,  that  this  last  named 
straight  scarp  may  actually  be  a  resultant  of  two  intersecting  systems,  such  as 
have  been  described. 

ZIGZAG    FAULT   SCARP   ON   TONOPAH-SODAVILLE   ROAD. 

On  the  north  side  of  the  main  road  which  leads  from  Tonopah  toward  Soda- 
ville,  in  the  western  part  of  the  area  mapped,  a  similar  phenomenon  was  noted. 
The  road  lies  in  a  depression,  on  the  south  side  of  which  there  is  an  irregular, 
undecided  embankment  consisting  mostly  of  fragmental  material  and  having  a 
height  of  about  10  feet.  On  the  north  side  there  is  a  sharp  scarp  about  25  feet 
high,  consisting  of  a  continuous  outcrop  of  solid,  glassy  Tonopah  rhyolite-dacite. 
On  inspection  this  scarp  shows  well-marked  rectilinear  courses,  forming  steps 


78  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

or  jogs,  although  the  detail  is  somewhat  rounded  by  erosion.  It  runs  chiefly  in 
two  directions— N.  60°-70°  E.  and  N.  30°-40°  W.  This  zigzag  course,  and  the 
absence  of  the  scarp  on  the  south  side  of  the  depression,  as  in  the  case  of  the 
occurrence  previously  described,  seem  to  indicate  a  complex  fault  fracture,  and 
the  directions  of  the  rectilinear  components  in  each  case  are  similar.  In  this 
case  also  the  indentations  are  not  due  to  gulches,  for  there  is  usually  not  the 
slightest  depression  at  the  top  of  the  scarp,  at  the  angles.  The  scarp  continues 
beyond  the  area  mapped.  The  general  trend  (being  the  resultant  of  the  two 
directions  noted)  is  almost  exactly  parallel  to  the  similar  scarp  previously  described. 

ORIGIN    OF   ZIGZAG    FAULT   SCARPS. 

From  the  general  sum  of  knowledge  concerning  the  relation  of  faulting  to 
topography  in  this  district  (see  p.  114),  it  is  inferred  that  probably  these  slight 
scarps  are  due  to  differential  erosion  and  mark  the  limits  of  fault  blocks  which  are 
slight^'  harder  than  those  contiguous.  Their  invariable  slight  relief  strengthens 
this  idea.  Similar  scarps,  which  have  been  proved  to  have  originated  in  this 
manner,  are  characteristic  of  fault  contacts  in  other  portions  of  the  area  mapped. 
The  other  possible  hypothesis  is  that  the  faults  are  recent,  and  that  the  scarps 
have  formed  as  a  result  of  direct  displacement  of  the  surface.  In  spite  of  the 
fact  that  the  probabilities  seem  to  favor  the  first  explanation,  certain  features 
support  the  second.  One  of  these  is  that  scarps  of  this  sort,  like  those  just 
described,  sometimes  have  on  each  side  material  belonging  to  the  same  formation, 
as  the  scarp  marked  B  in  PI.  XII,  which  has  tuff  on  both  sides,  or,  as  the  scarp 
last  described,  on  the  Tonopah-Sodaville  road,  which  has  the  glassy  Tonopah  rhyo- 
lite-dacite  on  both  sides.  If  these  surface  features  are  due  to  erosion,  the  higher 
block  must  be  slightly  harder  than  the  lower  and  must  represent  a  slightly  more 
resistant  part  of  the  formation.  This  indeed  is  true  in  the  place  last  mentioned, 
where  the  glassy  Tonopah  rhyolite-dacite  in  the  area  north  of  the  road  is  the  solid 
intrusive  lava,  while  the  formation  included  under  the  same  head  in  the  region 
south  of  the  road  is  surface  material,  breccias  and  tuffs,  and  therefore  more  fragile 
and  more  easily  eroded.  Another  circumstance  which  also  favors  the  idea  of 
direct  displacement  is  that  the  two  chief  compound  scarps  just  described  both 
face  the  south.  It  is  known  from  independent  evidence  that  the  southern  part  of 
the  area  mapped  has  been  downthrown  in  respect  to  the  northern  part,  so  that  a 
slight  continuation  of  the  general  movement  into  very  recent  times  might  result 
in  these  south-facing  scarps. 


FAULTING.  79 

ORIGIN   OF   ZIGZAG   FAULTS. 

Zigzag  fault  courses  like  those  described  may  originate  in  two  ways:  (1)  By 
the  intersection  of  independent  fault  systems  which  produce  a  zigzag  line  of  equal 
dislocation  oblique  to  both  the  intersecting  systems,  as  explained  in  the  considera- 
tion of  the  Wandering  Boy  fault  (pp.  157-161);  and,  (2)  by  a  simple  fault  whose 
initial  movement  follows  a  zigzag  course  along  previously  existing  fractures. 

INTRUSIONS  CONTROLLED  BY  INTERSECTING  FRACTURES. 

Rectilinear  boundaries  or  rectilinear  boundary  scarps  do  not  always  indicate 
faulting  in  the  sense  above  described,  where  one  of  the  rocks  is  intrusive.  A  case 
is  furnished  by  the  outline  of  the  Golden  Mountain  intrusion.  As  shown  on  the 
map,  the  contact  of  the  Golden  Mountain  dacite  with  the  earlier  andesite,  on  the  east 
side  of  Gold  Hill,  is  so  straight  as  to  suggest  the  possibility  of  faulting.  Moreover, 
east  of  Gold  Hill  the  long  south  contact  of  the  same  intrusion  follows  alternating 
straight  northwest-southeast  and  northeast-southwest  courses,  strongly  suggesting 
the  resultant  of  two  intersecting  systems  of  faults,  similar  to  the  scarps  already 
described.  But  excellent  evidence  that  the  contact  has  not  been  faulted  is  present 
in  the  band  of  dacite  glass  which  represents  the  quickly  chilled  lava  along  the 
margin  of  the  intrusion,  and  which  was  found  to  follow  the  contact  along  its  differ- 
ent courses. 

It  appears  that  the  straight  western  limit  of  the  intrusive  Brougher  dacite  along 
Gold  Hill,  above  referred  to,  has  been  determined  by  a  preexisting  fault,  for  the 
continuation  of  this  fault  is  evident  near  the  California-Tonopah  (California  fault), 
where  a  dike  from  the  main  dacite  mass  follows  the  fault  zone.  In  this  light,  also, 
it  seems  probable  that  the  rectilinear  courses  and  the  set-offs  regularly  in  the 
same  direction  on  the  south  side  of  the  Golden  Mountain  indicate  that  the  intrusive 
contact  was  here  also  determined  by  a  system  of  preexisting  intersecting  faults  or 
fractures. 

CORROBORATION  OF  CONCLUSIONS. 

A  number  of  faults  that  were  located  on  the  surface  by  the  methods  above 
given  were  subsequently  found  in  mine  workings  and  observed  more  closely  and 
satisfactorily.  The  Mizpah  fault  was  recognized  at  an  early  stage  in  the  investiga- 
tion, both  on  the  surface  and  underground.  The  Burro  fault,  distinguished  and 
followed  with  great  difficulty  at  the  surface,  was  subsequently  developed  under- 
ground. The  Wandering  Boy  and  Fraction  faults,  first  distinguished  on  the  surface, 
were  subsequently  found  to  be  well  exhibited  in  the  mine  workings. 


80  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,  NEVADA. 

ACCURACY  OF  FAULT  MAPPING. 

In  this  volcanic  region  faults  can  very  often  not  be  distinguished  at  all.  This 
is  the  case  if  similar  rocks  lie  on  both  sides  of  a  fault  and  other  signs  fail.  There- 
fore on  the  map  some  faults  have  been  projected  a  reasonable  distance  and  probable 
connections  made  across  spaces  intervening  between  different  fragments  of  what  is 
probably  a  single  fault  line.  While  the  structure  as  finally  depicted  is  undoubtedly 
not  strictly  accurate  in  many  details,  the  general  features  are  well  shown,  and  the 
error,  were  a  closer  study  possible,  would  undoubtedly  be  found  to  be  not  that  too 
many  faults  are  represented,  but  that  many  have  escaped  detection. 

FAULTING  DUE  TO  VOLCANIC  ACTION. 

The  faulting  in  this  district  is  of  extraordinary  interest,  for  the  origin, 
time,  and  cause  are  clearly  understood.  It  is  rare  that  any  explanation  other 
than  a  general  unsubstantiated  hypothesis  can  be  applied  to  any  particular  case 
of  faulting.  Here,  however,  it  is  plain  that  the  faulting  was  the  result  of 
adjustments  of  the  crust  to  suit  violent  migrations  of  volcanic  rock;  that  it 
originated  with  the  swelling  up  of  the  crust  and  its  forcible  thrusting  up  and 
aside  to  make  way  for  the  numerous  columns  of  escaping  lava;  and  that  after 
the  cessation  of  the  eruptions  it  was  continued  by  the  irregular  sinking  of  the 
crust  into  the  unsolid  depths  from  which  the  lavas  had  been  ejected.  It  can 
readily  be  seen  that  all  sorts  of  pressure  (from  below  upward,  lateral,  and 
downward,  by  virtue  of  gravity)  must  have  been  concerned  in  such  movements, 
and  that  the  first  faults  were  due  rather  to  upward  and  lateral  irregular  thrusts, 
while  the  later  ones  (in  many  cases  along  the  same  planes  as  the  first)  were 
due  to  gravity.  So  reversed  and  normal  faults  are  equally  natural,  and  both 
occur  frequently. 

APPLICATION   OF    PRINCIPLES   TO   REGIONS   LYING    BEYOND   AREA    MAPPED. 

These  observations  are  probably  not  of  slight  and  local  significance.  The 
faulting  is  intense,  and  the  faults  have  frequently  very  great  displacements, 
amounting  to  many  hundred  feet  at  least.  Moreover,  considerable  areas  are 
affected  by  subsidence  or  elevation  connected  with  and  in  part,  at  least,  accom- 
plished by  faulting,  as,  for  instance,  the  relative  depression  of  the  southern  part 
of  the  area  mapped  (near  the  dacite  necks),  as  compared  with  the  northern  portion. 
The  cause  of  these  larger  movements  is  plainly  the  same  as  that  of  the  individual 
faults.  Evidently  such  phenomena  are  not  confined  to  the  area  mapped,  but 
extend  indefinitely  beyond  it.  The  writer  at  first  looked  upon  the  faulting  at 
Tonopah  as  exceptional  and  local,  and  not  to  be  connected  with  ordinary 
faulting  in  the  Great  Basin;  but  there  now  appears  no  reason  for  doubting 


FAULTING.  81 

that  the  phenomena  within  this  small,  carefully  studied  area  are   typical   of  the 
unstudied  similar  volcanic  region  beyond  the  limits  of  the  map. 

The  individual  faults  have  been  shown  to  have  been  minor,  irregular 
movements  attending  broader  elevations  or  depressions;  and  the  hypothesis  has 
been  presented  that  at  an  earlier  period  the  lake  basin  in  which  the  Siebert 
tuffs  were  laid  down  was  formed  by  general  subsidence  of  an  area  that  was 
occupied  by  earlier  eruptive  rocks  (the  earlier  dacitic  eruptions)  and  that  this 
basin  was  destroyed  by  a  broad  uplift  which  preceded  the  later  dacitic  outbursts. 
There  is  little  doubt  that  these  earlier  movements  were  attended  by  some  faulting, 
although  such  faults  would  be  difficult  of  detection,  especially  in  the  presence  of 
the  subsequent  complicated  faulting  of  the  period  of  the  later  dacitic  intrusions. 

SUGGESTED   EXPLANATION   OF   GREAT   BASIN   TERTIARY   DEFORMATIONS. 

The  recognition  (pp.  52,  70)  of  the  facts  that  the  lake  in  which  the  white  tuffs 
were  laid  down  was  a  very  large  one,  and  that  it  very  likely  corresponds  to  the 
great  Miocene  Pah-Ute  Lake  of  King,  gives  a  broader  interest  to  this  hypothesis 
of  its  origin;  and  the  hypothesis  naturally  extends  itself  to  the  other  Tertiary 
lake  basins  which  preceded  and  followed  the  Pah-Ute. 

In  the  great  interior  province  in  which  Tonopah  is  situated,  and  which  lies 
between  the  Wasatch  and  the  Colorado  Plateau  on  the  east  and  the  Sierra  Nevada 
on  the  west,  a  number  of  successive  lake  basins  of  van-ing  extent  formed  during 
the  Tertiary,  as  was  first  shown  by  King.  These  changing  basins,  of  varying 
shape  and  extent,  were  due  to  uneasy  continual  warpings  (elevations  and  depressions) 
which  continued  through  the  Tertiary  period  down  to  the  present  day.  This 
warping  has  been  contemporaneous  with  folding  and  faulting,  and  all  of  this 
crustal  disturbance  has  been  accompanied  by  volcanism. 

"In  general  the  period  of  deformation  which  lasted  from  the  Mesozoic  to  the 
present  has  been  contemporaneous  with  volcanic  activity.  By  far  the  most  energetic 
vulcanism,  so  far  as  we  know,  occurred  in  the  Tertiary,  beginning  probably  in  late 
Cretaceous  or  early  Eocene  and  extending  into  the  Pleistocene.  Vulcanism  and 
deformation  were,  therefore,  allied  phenomena."" 

In  the  earlier  recognition  of  this  coextension  of  the  two  phenomena  of  deforma- 
tion and  volcanism  the  writer's  conception  was  that  they  were  both  the  result  of  a 
single  unknown  cause.  In  the  light  of  the  Tonopah  studies,  however,  it  seems  fair 
to  admit  that  the  former  may  have  been  the  result  of  the  latter,  the  effect  of  the 
repeated  accumulation  and  eruption  of  vast  bodies  of  molten  material,  and  the  sub- 
sequent subsidences  and  local  adjustments. 

aSpurr,  J.  E.,  Origin  and  structure  of  the  Basin  ranges:  Bull.  Geol.  Soc.  America,  vol.  12,  p.  248. 
16843— No.  42—05 6 


82  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,   NEVADA. 

CONTINUANCE   OF   VOLCANIC    EPOCH. 

Viewed  in  this  or  in  other  lights,  there  is  small  reason  for  believing  that  the 
period  of  volcanism  in  this  province  is  past.  It  rather  appears  that  we  are  still  in 
it.  The  occurrence  of  recent  almost  undefaced  basaltic  craters  at  various  points, 
such  as  at  Silver  Peak  (PI.  XV,  A),  at  Lake  Mono,  and  in  central  Oregon,  show 
that  the  last  eruptions  occurred  only  a  few  hundred  years  ago,  while  the  evidence  of 
enormous  Pleistocene  and  recent  elevation  and  subsidence,  especially  in  the  western 
part  of  the  region,  near  the  Sierra  Nevada,"  suggests  the  migrations  of  the  molten 
tide  beneath  the  present  crust. 

a  Spun,  J.  E.,  op.  cit.,  p.  247,  248;  also  Bull.  U.  S.  Geol.  Survey  No.  208,  pp.  110, 129,  209,  210,  etc. 


CHAPTER    II. 
MINERAL  VEINS. 

VEINS  OF  THE  EARLIER  ANDESITE. 
PERIOD  OF  MINERALIZATION. 

The  most  important  veins  of  the  Tonopah  district  occur  in  the  earlier  andesite 
and  do  not  extend  into  the  overlying  rocks;  hence,  where  the  earlier  andesite -is 
not  exposed  at  the  surface  the  later  rocks  form  a  capping  to  the  veins,  and  this 
capping  must  be  passed  through  before  anything  can  be  learned  of  the  presence 
or  the  nature  of  the  veins  beneath.  This  fact  shows  pretty  plainly  that  the  veins 
were  deposited  before  the  eruption  of  the  later  andesite  and  immediately  after 
that  of  the  earlier  andesite,  for  the  period  of  erosion  between  the  two  andesites 
seems  to  have  exposed  the  veins  at  the  surface,  indicating  that  they  were  formed 
before  this  period  or  early  in  it.  Indeed,  there  is  every  evidence  that  the  veins 
were  formed  by  ascending  hot  waters  succeeding  and  connected  with  the  earlier 
andesite  intrusion,  and  that  these  waters  had  become  inactive  by  the  time  of  the 
later  andesites. 

NATURE  OF  CIRCULATION  CHANNELS. 

The  openings  which  afforded  channels  for  these  ascending  waters  were  of  the 
nature  of  sheeted  zones.  The  rock  was  complexly  fractured,  apparently  soon 
after  cooling,  and  probably  as  a  result  of  the  stresses  exerted  by  the  still  active 
volcanic  energy  below.  A  major  set  of  fractures  extended  in  an  east-west  direc- 
tion and  zones  of  close-set  parallel  fractures  attained  a  maximum  thickness  of 
several  feet.  These  became  the  chief  channels  of  circulation.  In  places  the 
circulating  waters  divided  into  separate  channels,  which  diverged  and  frequently 
reunited,  and  many  lateral  channels  were  favorable  to  egress  of  the  waters. 
These  channels,  however,  were  apt  to  get  poorer  as  the  distance  from  the  main 
fracture  zone  increased. 

The  conditions  above  stated  are  clearly  shown  by  a  study  of  the  veins  of 
Mizpah  Hill  and  vicinity  (fig.  12).  The  circulation  channel  now  occupied  by  the 
Mizpah  vein  may  be  taken  as  a  type  of  the  main  fracture  zones,  and  the  diverging 
Burro  veins,  dwindling  as  they  increase  their  distance  from  the  master  veins, 
represent  the  lateral  channels.  The  splitting  and  reuniting  is  shown  by  the 

structure  of  the  veins  at  many  points. 

83 


84 


GEOLOGY    OF   TONOPAH   MINING    DISTRICT,  NEVADA. 
VEINS  DUE  CHIEFLY  TO   REPLACEMENT. 


That  the  circulation  channel  was  in  practically  every  case  a  fracture  zone  and 
not  an  open  fissure  is  shown  by  the  study  of  the  veins,  which  reveals  all  stages 
in  the  change  from  a  fracture  zone  in  porphyry  to  a  solid  quartz  vein.  In  many 
cases  the  vein  consists  simply  of  a  zone  of  more  or  less  altered  andesite,  not 
essentially  different,  except,  perhaps,  for  a  somewhat  greater  silicification,  from 


N 

I  > 


North  Star  Shaft  a 


•  Tonopah  Extension  Shaft 


i  MacNamara  Shaft 


Montana  Tonopah  Shaft B 


Tonopah  Mining  Company 
a  Main  Shaft 


:.'lzpah  Vein 


•  West  End  Shaft 


BFractlon  No.l  Shaft 
•Fraction  No.2  Shaft       x^y 

B  Wandering  Boy  Shaft 

•  Fraction  NoJ  Shaft 


B  Silver  Top  Shaft 
»  Stone  Cabin  Shaft 

Valley  View 
Vein  Group 


•  Gold  Hill  Shaft 


•  Tonopah  City  Shaft 


300   200  100  0 


California 

Tonop.'ih 
"   Sh«ft_ 


FIG.  12. — Map  showing  outcropping  veins  of  Tonopah. 

the  andesite  which  forms  the  walls.  This  zone  is  cut  by  parallel  fractures  having 
the  same  strike  and  dip  as  the  walls,  and  the  walls  themselves  are  nothing  more 
than  stronger  fractures  of  the  same  kind.  In  the  next  stage,  where  part  of  this 
fractured  zone  becomes  altered  to  quartz,  the  main  wall  fractures  have  been  the 
most  favorable  for  water  circulation,  so  that  sometimes  a  hanging-wall  streak  of 
quartz  and  a  foot-wall  streak  are  found  with  only  altered  andesite  between. 


I- 
;: 


VEINS    OF    THE    EARLIER    ANDESITE.  85 

Sometimes,  also,  either  the  hanging-wall  or  the  foot-wall  streak  may  be  wanting. 
Next,  -streaks  of  quartz  parallel  with  the  walls  may  be  found,  or  the  quartz  may 
form  a  network  in  the  andesite.  Thus  the  process  may  be  traced  to  the  stage 
where  the  whole  of  the  andesite  is  replaced  by  quartz,  forming  a  solid  vein 
several  feet  in  width.  As  a  rule,  however,  more  or  less  decomposed  andesite 
forms  part  of  the  vein. 

PORTIONS  OF  VEINS  DUE  TO  CAVITY  FILLING. 

As  exceptions  there  are  found  streaks  of  quartz,  usually  small,  within  the 
vein,  which  show  crustification  and  comb  structure  and  thus  bear  evidence  of 
having  been  formed  in  cavities.  These  cavities,  however,  were  often  of  irregular 
shape  and  were  not  fissures,  properly  speaking,  but  spaces  of  dissolution,  and 
were  the  effect  of  the  mineralizing  waters  themselves. 

The  largest  example  of  a  crustified  vein  is  found  in  certain  parts  of  the 
Montana  Tonopah  workings,  where  the  cavities  were  sometimes  2  or  3  feet  in 
diameter  and  gave  rise  to  well-banded  ores  (PL  XIII). 

CROSS  WALLS  AND  ORE  SHOOTS. 

The  fractures  transverse  to  the  main  system  had  a  not  inconsiderable  effect 
in  determining  the  course  of  the  ore  solutions.  Along  important  transverse 
fractures  it  has  been  found  that  the  vein  frequently  widens  or  narrows  abrupth*, 
the  cross  fractures  playing  the  same  part  as  the  lateral  wall  fractures,  even  if  not 
to  such  an  extent,  and  so  earning  the  name  of  cross  walls  which  has  been  given 
them.  To  these  cross  walls,  more  or  less  pronounced,  the  division  of  the  water 
circulation  along  the  main  zone  into  columns  of  unequal  importance  was  due,  and 
hence  the  mineralization  accomplished  by  these  waters  was  correspondingly 
localized.  It  is  probable  that  the  recognized  ore  shoots  or  bonanzas  had  their 
origin  in  this  way. 

NATURE  OF  MINERALIZING  AGENTS. 

That  the  mineralizing  agent  was  water  is  evident  from  the  character  of  the 
vein  and  from  the  nature  of  the  alteration  of  the  wall  rock.  That  its  action  was 
probably  connected  with  the  earlier  andesite  eruption  is  shown  by  the  fact  that 
it  followed  this  and,  at  least  so  far  as  mineralizing  activity  was  concerned,  was 
of  limited  duration,  for  its  effects  have  not  been  determined  in  the  succeeding 
later  andesite.  It  appears  probable,  therefore,  that  the  mineralizing  agents  were 
volcanic  waters,  such  as  are  usually  among  the  after  effects  of  volcanic  outbursts, 
and  that  they  were  hot  and  ascending.  A  consideration  of  their  effects,  as  dis- 
played both  in  the  veins  and  in  the  country  rock,  will  throw  further  light  on 
their  nature. 


86  GEOLOGY   OF   TONOPAH    MINING    DISTRICT,  NEVADA. 

PRIMARY  ORES. 
LOCALITY. 

The  contents  of  veins  lying  near  the  surface  have  been  transformed  more  or 
less  into  new  minerals — minerals  that  are  more  stable  under  surface  conditions; 
the  materials  originally  deposited  from  the  mineralizing  solutions  must  therefore 
be  sought  in  the  unoxidized  lower  region.  The  Montana  Tonopah  veins  carry 
solid  sulphide  ores,  primary  and  contemporaneous  with  the  original  quartz  gangue 
and  very  slightly  altered,  presenting  strong  contrast  with  the  oxidized  ores  of 
the  Mizpah  mine.  Similar  sulphide  ores  have  been  found  in  the  North  Star,  the 
Tonopah  Extension,  the  Midway,  and  the  Tonopah  and  California. 

COMPOSITION. 

MINERALS. 

Quartz. — In  these  veins  the  chief  gangue  mineral  is  quartz,  frequently  well 
crystallized  and  translucent,  but  more  usually  rather  fine-grained  and  dense,  and 
mixed  with  more  or  less  aluminous  material.  This  material,  which  will  be  described 
later  on,  is  a  residue  of  the  least  soluble  material  of  the  earlier  andesite.  Under 
the  microscope  the  quartz  has  a  characteristic  structure,  distinct  from  that  of 
ordinary  crystalline  vein  quartz.  Instead  of  the  coarse  interlocking  grains  com- 
monly displayed  by  vein  quartz,  these  veins  usually  show  a  mosaic  in  which  the 
grain  varies  enormously  in  size,  ranging  from  very  fine  cryptocrystalline  to  very 
coarse.  Under  the  microscope  the  aluminous  material  proves  to  be  very  fine 
muscovite  (sericite).  The  quartz  holds  numerous  fluid  inclusions,  which  contain 
bubbles,  showing  that  the  included  material  was  in  a  state  of  vaporous  tension 
at  the  time  of  its  inclusion  or  at  the  time  of  the  vein  formation,  and  that  it  has 
contracted  so  as  to  fill  only  part  of  its  original  chamber  upon  the  lowering  of 
the  temperature.  The  inclusions  are  frequently  densely  packed  and  curiously 
arranged.  In  some  cases  the  interior  of  the  crystals  is  clear,  while  the  marginal  zone 
is  packed  with  inclusions.  Frequently  the  quartz  has  the  rough  retiform  structure 
which  is  due  to  the  intergrowth  of  idiomorphic  crystals  starting  from  independent 
crystallization  centers,  and  which  is  often  characteristic  of  quartz  formed  by 
replacement."  There  are  also  coarser  veinlets  of  quartz,  later  than  the  bulk  of 
the  vein,  which  were  introduced  along  cracks,  and  these  in  places  show  comb 
structure. 

Adularia. — The  nearly  pure  potash  feldspar,  adularia,  a  purer  variety  of 
orthoclase,  is  a  common  gangue  mineral.  It  is  frequently  very  abundant,  usually 
in  more  or  less  idiomorphic  crystals  that  show  the  characteristic  rhombic  cross- 
section.  It  is  intercrystallized  with  the  quartz,  which  often  incloses  isolated 

aSpurr,  J.  E.,  Mon.  U.  8.  Oeol.  Survey,  vol.  81,  p.  218. 


PRIMARY    ORES    OF    THE    EARLIER    ANDESITIC    VEINS.  87 

idiomorphic  crystals  of  it,  showing  the  nearly  contemporaneous  deposition  of  the 
two  minerals.  Its  condition  is  fresh  and  glassy,  and  only  when  it  has  been  locally 
strained  does  it  show  cleavage  cracks.  That  it  has  been  deposited  from  solution  in 
the  same  way  as  the  quartz  and  the  metallic  minerals  of  the  veins  is  evident.  Where 
the  adularia  and  quartz  crystallize  together  the  sharply  idiomorphic  feldspar, 
included  in  the  xenomorphic  quartz,  shows  the  former  to  have  first  crystallized,  the 
order  being  the  same  as  in  igneous  rocks.  The  adularia,  like  the  quartz,  is  sometimes 
closely  packed  with  liquid  and  gaseous  inclusions. 

For  chemical  determination  a  specimen  (No.  254)  from  the  Fraction  vein, 
which  is  made  up  of  this  mineral  and  quartz,  finely  intercrystallized,  was  ground. 
The  quartz  was  then  removed,  as  far  as  possible,  by  the  use  of  the  Thoulet 
solution.  The  best  material  thus  obtained  was  analyzed  by  Dr.  W.  F.  Hillebrand 
of  the  United  States  Geological  Survey. 

Analysis  of  adularia  and  quartz. 
SiOj  ......................................................................  75.28 

Al2O3a  ....................................................................  13.19 

Na,O  ......................................................................  32 

K2O  ......................................................................  10.95 

99.74 

Inspection  of  this  analysis  shows  that  the  material  is  a  nearly  pure  silicate 
of  aluminum  and  potassium,  which,  from  its  optical  properties,  can  be  only 
orthoclase  or  adularia.  The  silica,  however,  is  considerably  too  high,  showing  a 
mixture  of  quartz.  By  calculating  the  amount  of  silica  needed  for  orthoclase  it 
is  found  that  about  28.8  per  cent  of  it  is  present  as  free  quartz,  leaving  as 
components  of  the  adularia  — 

SiO2  ......................................................................  46.48 

A12O,  .....................................................................  13.19 

NajO  ......................................................................  32 

K20  ......................................................................  10.95 


70.94 
Recalculating  this  on  a  basis  of  100  we  have  — 

Si02  .....................................................................    65.52 

A12O,  ....................................................................     18.59 

Na,0  ..........................  '  ...........................................  45 

K20  ...............................................  15.44 


100.00 

Sericite. — Muscovite  occurs  in  the  veins  only  as  a  fine  aggregate  (sericite). 
It  usually  is  scattered  through  the  vein,  or  is  irregularly  bunched  in  certain 
areas.  It  has  been  found  included  in  adularia. 

a  May  contain  traces  of  FejO3,  etc. 


88  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 

Carbonates. — A  carbonate  is  sometimes  found  microscopically  mingled  with 
the  quartz  as  a  gangue  material,  and  has  also  been  noted  macroscopically.  Doctor 
Hillebrand  has  determined  that  this  is  composed  of  the  carbonates  of  lime,  iron, 
magnesia,  and  manganese,  in  the  proportions  stated  later  on. 

Silver  sulphides. — The  principal  metallic  mineral  of  the  ores  is  a  black  sulphide, 
usually  dense,  fine  grained,  and  intimately  intermingled  with  quartz.  As  seen 
under  the  microscope,  this  black  sulphide  has  a  typical  blue-black  color,  and 
often  shows  cleavage,  but  almost  always  lacks  crystal  outlines.  In  tiny  cavities, 
however,  crystals  form.  These  are  usually  the  six-sided,  tabular,  striated  crystals 
characteristic  of  polybasite  and  stephanite.  Partial  analysis  by  W.  T.  Schaller 
of  such  crystals  from  the  Montana  Tonopah — crystals  which  may  possibly  be 
secondary  (see  p.  95) — showed  appreciable  amounts  of  antimony  and  copper,  the 
latter  ingredients  indicating  that  the  mineral  is  polybasite  rather  than  stephanite. 
In  such  cavities  argentite  crystals  also  occur. 

Silver  chloride. — What  is  apparently  silver  chloride  (cerargyrite)  is  found  in 
some  of  the  primary  ores,  interwoven  with  the  primary  sulphides  in  such  a  way 
as  to  seem  to  denote  contemporaneous  crystallization.  In  thin  sections  of  such 
ores  the  chloride  is  apt  to  be  more  or  less  bunched,  as  is  the  sulphide,  but  the 
two  are  occasionally  intergrown,  with  clear-cut  lines  of  demarcation,  seeming  to 
denote  independent  and  contemporaneous  origin. 

Chalcopyrite. — Chalcopyrite  in  occasional  small  grains  is  often  noted  in  the 
primaiy  ores,  and  is  frequently  so  intergrown  with  the  primary  silver  sulphide 
and  with  the  gangue  minerals  as  to  indicate  its  primary  character.  In  quantity, 
however,  it  is  relatively  unimportant. 

Pyrite. — Pyrite  in  the  veins  is  comparatively  scanty,  much  more  so  than  in 
the  wall  rock.  In  many  thin  sections  of  the  ores  it  is  not  found  at  all;  in  others 
it  occurs  in  considerable  amount.  In  the  primary  ores  it  is  frequently  intergrown 
with  the  silver  sulphide,  with  which  it  is  evidently  contemporaneous,  though 
usually  less  in  quantity. 

Galena. — Galena  has  been  noted  in  the  high-grade  sulphide  ores  of  the 
Montana  Tonopah,  where  it  is  associated  with  silver  sulphides,  chalcopyrite,  and 
pyrite.  A  picked  specimen  from  the  460-foot  level  which  contained  galena  was 
analyzed  for  the  Survey  by  R.  H.  Officer  &  Co.,  of  Salt  Lake  City,  and  showed 
8.9  per  cent  lead,  5.08  per  cent  silver  (1,481.8  ounces  per  ton),  and  38.26 
ounces  gold. 

Blende. — What  is  probably  zinc  blende  has  been  detected  microscopically  by 
the  writer  in  the  primary  ore  of  the  Midway  shaft.  Zinc  sulphide  has  been 
detected  chemically  in  the  Montana  Tonopah  primary  ores. 


PRIMARY    SULPHIDE    ORES.  89 

Gold. — Gold  is  present  in  the  average  ore  in  the  proportion  of  gold  to  silver 
of  1:100  by  weight.  It  has  never  been  detected  by  the  eye  in  the  sulphide  ores, 
either  in  the  hand  specimen  or  under  the  microscope,  though  it  has  been  found  in 
metallic  particles  both  macroscopically  and  microscopically  in  the  oxidized  ores. 

ANALYSIS   OP    PRIMARY    SULPHIDE   ORES. 

Picked  samples  of  rich  primary  sulphide  ore  were  taken  from  the  Montana 
vein  of  the  Montana  Tonopah  mine  at  depths  ranging  between  -160  and  512  feet. 
These  were  crushed  and  the  sulphides  were  concentrated  by  panning.  The  analysis 
of  the  concentrates  by  Dr.  W.  F.  Hillebrand  of  the  U.  S.  Geological  Survey,  is  as 
follows: 

Analysis  of  concentrates  of  primary  sulphide  ore  from  Montana  Tonopah  mine. 

Per  cent. 
Siliceous  matter 15. 18 

Gold 82 

Silver 25.92 

Lead t 6.21 

Copper 1 . 32 

Iron 9. 87 

Manganese 1. 36 

Zinc 5.  84 

Selenium 2. 56 

Tellurium None. 

Arsenic 19 

Antimony 92 

Magnesia 1. 49 

Lime 3.  70 

Carbon  dioxide 6. 34 

Sulphur Not  det 


81.  72 
The  composition  of  the  carbonate  is  as  follows: 

Per  cent  (in  terms 
of  u  hole  analysis). 

Lime  carbonate  (CaCO,) 6.71 

Magnesia  carbonate  (MgCO3) 3. 13 

Iron  carbonate  ( FeCO3) 2. 36=Fe  =1. 14 

Manganese  carbonate  (MnCOs) 2.57=Mn=1.32 

The  whole  of  the  manganese,  therefore,  exists  as  carbonate. 
Doctor  Hillebrand  remarks: 

"Prolonged  boiling  with  hydrochloric  acid  decomposed  all  the  sulphide  except 
pyrite  (and  chalcopyrite  if  present).  Hot  dilute  nitric  acid  then  dissolved  the  pyrite 
and  also  considerable  selenide  of  silver  (and  copper?).  The  residue  remaining  after 
this  treatment  consisted,  aside  from  quartz,  of  very  malleable  black  scales  and  parti- 


90  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,   NEVADA. 

cles  which  showed  under  the  microscope  the  corroding  action  of  the  reagents  used. 
When  boiled  with  concentrated  nitric  acid,  these  black  particles  became  golden  in  color, 
and  the  solution  contained  little  or  no  selenium,  but  of  this  last  I  am  not  positive. 
So  far  as  can  be  judged,  the  whole  of  the  gold  exists  in  the  form  of  this  malleable 
black  alloy,  which  is  so  high  in  silver  that  the  latter  can  all  be  extracted  by  strong 
nitric  acid.  The  cause  of  the  black  color  is  not  apparent,  and  it  puzzles  me  not  a 
little." 

SUMMARY   OF  VEIN   MINERALS. 

The  principal  minerals  of  the  primary  veins  are,  then,  quartz,  adularia,  and 
some  sericite,  carbonates  of  lime,  magnesia,  iron,  and  manganese,  sulphides  of 
silver,  antimony,  copper,  iron,  lead,  and  zinc  (sulphides  occurring  in  the  form  of 
argentite,  stephanite,  polybasite,  chalcopyrite,  pyrite,  galena,  and  blende),  silver 
selenide,  and  gold  in  a  yet  undetermined  form.  The  remarkable  thing  about  the 
metallic  contents  is  the  scarcity  of  the  common  elements  and  the  abundance  of  the 

rare  ones. 

OXIDATION. 

The  chief  alteration  of  the  rocks,  as  will  hereafter  be  explained,  is  due  to 
the  action  of  ascenaing  underground  waters.  The  effects  of  descending  surface 
waters  are  seen  chiefly  in  oxidation  and  similar  processes  acting  upon  the  altered 
rocks.  The  oxidation  or  other  alteration  of  metallic  sulphides  is  the  chief  change, 
and,  on  account  of  the  universal  presence  of  pyrite  formed  by  hot-spring  action, 
this  change  can  be  observed  both  in  the  veins  and  in  the  country  rocks. 

DEPTH    OF   OXIDATION. 

The  depth  to  which  this  oxidation  of  pyrite  has  penetrated  is  exceedingly 
irregular,  being  quite  different  in  neighboring  shafts,  and  is  very  variable  in 
different  parts  of  the  same  workings.  The  difference  plainly  depends  on  the 
porosity  and  fracturing  of  the  rock.  Where  these  are  greatest  the  oxidizing 
waters  have  penetrated  farthest  downward.  Along  veins  the  oxidation  generally 
penetrates  much  deeper  than  in  the  rock,  so  that  the  ores  may  be  oxidized  while 
the  country  rock  is  pyritiferous.  This  is  plainly  due  to  the  greater  rigidity  and 
brittleness  of  the  vein  as  compared  with  the  rock,  so  that  it  has  been  more 
fractured  by  .strains,  and  therefore  offers  a  readier  channel.  Even  in  veins  the 
depth  of  oxidation  is  very  irregular,  dependent  upon  the  amount  of  fracturing. 

CAP   ROCKS  AS   PROTECTION   FROM   OXIDATION. 

The  veins  which  outcrop  are  most  deeply  oxidized,  as  the  Mizpah  and  Valley 
View  veins.  The  former  is  for  the  most  part  oxidized  down  to  a  depth  of  nearly 
700  feet;  the  latter  is  oxidized  at  the  lowest  level  developed  (about  500  feet).  At 
a  depth  of  400  feet  in  both  mines  the  vein  is  almost  completely  oxidized  or 


OXIDATION    AND   CHLOBIDATION.  91 

otherwise  altered  by  surface  waters,  while  at  300  feet  and  below,  in  the  Valley 
View,  the  pyrite  in  the  country  rock  is  usually  unaltered. 

Where  veins  do  not  outcrop,  but  are  covered  with  a  blanket  of  overlying 
rock,  there  is  usually  comparatively  little  oxidation.  The  ore  in  the  Fraction,  at 
a  depth  of  a  little  over  200  feet,  is  a  sulphide  ore;  in  this  case  the  vein  has  been 
protected  by  a  covering  of  soft  volcanic  rock  (Fraction  dacite).  Similarly,  heavy 
sulphide  ores  were  found  in  the  Montana  Tonopah  at  a  depth  of  about  460  feet, 
the  veins  of  this  mine  apexing  under  the  later  andesite,  which  is  decomposed 
and  not  readily  susceptible  of  fracturing.  The  depth  of  general  oxidation  of  the 
country  rock  is  only  about  90  feet  in  the  Montana  Tonopah  shaft,  between  115 
and  185  feet  in  the  Wandering  Boy,  and  a  little  over  200  feet  in  the  Stone 
Cabin.  In  the  \Vandering  Boy  the  vein  is  oxidized  on  the  300-foot  level,  while 
the  country  rock  is  unoxidized. 

A  single  fracture  line  often  locally  divides  the  oxidized  from  the  unoxidized 
ore  and  rock.  This  line  of  demarcation  frequently  coincides  with  a  fault  line,  on 
which  account  it  was  suspected  that  some  of  the  oxidation  might  be  earlier  than 
the  faulting;  but  other  considerations  render  it  more  probable  that,  by  faulting, 
rocks  of  different  degrees  of  porosity  and  permeability  are  brought  together  and 
thus  the  result  is  accomplished. 

SILVER   CHLORIDE    IN    OXIDIZED  ZONE   OF   VEINS. 

In  the  ores,  the  effects  of  oxidation  are  to  change  pyrite  to  limonite,  and 
also  to  deposit  wad  (oxide  of  manganese),  which  is  formed  from  the  manganese 
carbonate  in  the  primary  ores;  while  horn  silver  (cerargyrite)  becomes  plentiful. 
This  abundance  of  horn  silver,  being  characteristic  of  the  oxidized  zone,  is  evidently 
due  to  the  effects  of  chlorine  contained  in  the  surface  waters.  Silver  bromides 
and  iodides  also  sometimes  accompanj-  the  chloride.  Free  gold  has  been  deposited. 

The  large  quantities  of  the  haloid  metallic  compounds  in  the  weathered 
portions  of  veins  in  the  desert  regions  of  America  have  been  especially  discussed 
by  Prof.  R.  A.  F.  Penrose,  jr.,"  who  suggests  that  they  are  probably  due  to 
the  arid  climate  which  has  prevailed  in  the  present  and  during  the  more  recent 
geologic  periods,  and  which  has  rendered  the  scanty  ground  waters  saline.  It  is 
suggested  that  these  saline  waters  have  accomplished  this  alteration. 

At  Tonopah  it  is  regarded  as  probable  that  the  primary  ore  contains  some 
silver  chloride,  and  it  is  possible  that  the  chloride  therein  contained  may  have 
been  concentrated  in  the  zone  of  weathering,  and  may  also  have  contributed  to 
the  predominance  of  chlorides  in  this  zone. 

ojour.  Geol.,  vol.  2,  p.  314. 


92  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,  NEVADA. 

ANALYSIS   OF   OXIDIZED   ORE. 

Concentrates  from  a  picked  sample  of  thoroughly  oxidized  ore  from  the 
300-foot  level  of  the  Valley  View  vein  were  found  by  Doctor  Hillebrand  to  have 
the  following  compositipn: 

Analysis  of  oxidized  ore  from   Valley   View  vein. 

Siliceous  matter 16. 53 

Gold 62 

Silver "62.54 

Lead 32 

Copper ." &.09 

Iron '. 1.39 

Manganese .07 

Zinc - 10 

Selenium .78 

Tellurium None. 

Arsenic .03 

Antimony -15 

Sulphur Not  -det. 

Total 82.62 

Concerning  this  analysis  Doctor  Hillebrand  adds: 

"After  extraction  of  all  the  silver  chloride  by  ammonia  the  residue  was  boiled 
with  hydrochloric  acid  until  silver  no  longer  appeared  in  the  filtrates.  The  insoluble 
matter  then  consisted,  aside  from  gangue,  of  a  little  pyrite,  of  the  same  black  gold- 
silver  alloy  found  in  the  unoxidized  ore,  and  of  a  pyritic-looking  mineral,  which 
latter  yielded  to  dilute  nitric  acid  much  silver  and  some  selenium,  leaving  a  residue 
of  gold." 

COMMENT   ON   THE   ORE   ANALYSES. 

Aside  from  the  complex  carbonate  of  lime,  manganese,  magnesia,  and  iron, 
the  analysis  of  the  primary  sulphide  ore  indicates  (p.  89)  the  presence  of  a  large 
amount  of  silver  sulphide — argentite.  Antimonial  sulphides  of  silver,  polybasite, 
very  likely  stephanite,  and  smaller  amounts  of  galena,  blende,  pyrite,  and 
chalcopyrite  are  also  indicated.  Of  very  great  interest  is  the  presence  of  a 
considerable  amount  of  selenium,  which  occurs,  in  part  at  least,  as  a  silver 
selenide,  and  the  absence  of  its  usually  closely  associated  element  tellurium. 
The  chemical  form  of  the  gold  is  3ret  uncertain. 

It  is  fair  to  assume  that  the  oxidized  ore  in  its  primary  sulphide  state  may  have 
had  a  composition  somewhere  relatively  near  that  of  the  primary  sulphide  analyzed. 
The  two  analyses  may  then  be  compared  with  the  object  of  perceiving  the  changes 
effected  by  oxidation.  There  is  no  element  which  can  be  considered  as  having 

a  38.10  as  lulpbldeg;    24.44  us  chloride,  selenitic,  and  alloy.  6  Mostly  oxidized. 


OXIDATION.  93 

remained  quantitatively  unaffected  during  oxidation,  so  that  merely  the  large  rela- 
tions can  be  glanced  at.  All  the  metals  except  silver  and  perhaps  gold  are  present 
in  the  oxidized  ore  in  much  diminished  proportions.  The  lead,  copper,  and  zinc  are 
present  in  small  quantities.  The  manganese  is  now  in  the  form  of  oxide,  but  very 
little  remains;  the  iron  is  in  the  form  of  oxide,  with  some  residual  or  secondary 
pyrite.  There  is  much  less  gold  in  proportion  to  silver  in  the  oxidized  ore  than  in 
the  sulphide  ore;  but  this  may  be  fortuitous  and  depend  on  the  specimen  selected. 
More  than  half  the  silver  is  in  the  form  of  sulphide,  and  from  the  very  small  quantity 
of  arsenic  and  antimony  present  this  portion  must  be  nearly  all  in  the  form  of 
argentite.  The  antirnonial  silver  sulphide  is  very  probably  pyrargyrite  (ruby  silver), 
judging  from  microscopic  observations.  It  is  noteworthy  that  antimony  and  arsenic 
are  present  in  the  same  proportions  to  one  another  in  both  analyses.  There  is  less 
than  a  third  as  much  selenium  in  the  oxidized  ore  as  in  the  sulphide  ore,  but  the 
discrepancy  is  not  so  great  as  in  the  case  of  lead,  copper,  manganese,  zinc,  arsenic, 
and  antimony;  and  this  selenium  seems  to  be  still  in  the  form  of  a  silver  selenide. 

Therefore  it  is  probable  that  during  the  process  of  oxidation  the  primary 
carbonates  were  attacked  by  surface  waters,  and  the  lime  and  magnesia,  together 
with  most  of  the  iron  and  manganese,  removed  in  solution.  Some  of  the  iron  and 
manganese  remain  as  oxides.  No  important  change  in  the  amount  of  gold  and  silver 
is  proved.  The  argentite  has  largely  remained  unaltered,  but  the  polybasite  (and 
stephanite  if  present)  has  probably  been  attacked,  and  much  of  the  silver  selenide. 
Part  of  this  silver  has  been  reprecipitated  with  little  change  of  position  as  secondary 
argentite,  not  distinguishable  from  the  primary  argentite,  while  a  large  portion  has 
been  altered  to  chloride  by  the  action  of  chlorine  contained  in  the  shallow 
underground  waters.  Most  of  the  arsenic  and  antimony  in  the  original  polybasite 
and  stephanite  has  been  removed  in  solution;  the  rest  goes  to  form  the  secondary 
sulphide  pyrargyrite,  as  indicated  by  numerous  field  observations.  The  pyrite  and 
the  chalcopyrite  have  been  attacked.  Most  of  the  iron  in  these  sulphides  has  been 
removed;  a  small  part  remains  as  oxide,  or  rarely  as  residual  or  secondary  pyrite. 
Nearly  all  the  copper  has  been  removed,  a  little  remaining  in  the  probable  form  of 
oxide. 

It  is  thus  seen  that  the  so-called  oxidized  ore  of  the  Tonopah  district,  like 
that  of  man}'  other  deposits  in  desert  regions,  is  really  a  modified  ore  consisting 
of  an  intimate  mixture  of  original  sulphides  (and  selenides),  together  with 
secondary  sulphides,  chlorides,  and  oxides.  This  case  is  without  doubt  character- 
istic of  the  whole  zone  of  oxidation  from  the  outcrop  downward,  for  the  ores 
throughout  the  zone  are  identical  microscopically. 

As  to  the  reprecipitation  lower  down  of  materials  dissolved  in  the  process 
of  oxidation  there  is  little  light.  The  plainly  secondary  sulphides  within  the 


94  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

sulphide  zone  are  argentite  and  pyrargyrite,  the  latter  always  coating  cracks  or 
cavities,  with  probably  chalcopyrite.  Possibly  the  copper  of  the  secondary 
chalcopyrite  is  formed  by  the  action  of  copper  solutions  from  above  on  primary 
pyrite,  but  galena  or  blende  have  not  been  noted  as  secondary  sulphides,  and  at 
best  are  rare.  Moreover,  the  secondary  silver  minerals  argentite  and  pyrargyrite 
are  more  abundant  than  secondary  pyrite  and  chalcopyrite,  and  all  these  usually 
occur  on  cracks  in  rich  primary  sulphides  and  not  in  barren  or  low-grade  ore, 
suggesting  the  derivation  of  the  secondary  minerals  from  this  rich  ore  by  lateral 
secretion  rather  than  an  exotic  origin. 

FORMATION   OF   GYPSUM    BY   OXIDIZING   WATERS. 

Gypsum  frequently  occurs  as  veinlets  or  incrustations  in  both  the  earlier  and 
the  later  andesites  where  these  are  altered.  It  is  more  rare  in  the  earlier 
andesite,  which  has  become  highly  silicified,  and  is  abundant  in  the  later  andesite, 
which  has  developed  a  large  amount  of  calcite  as  a  decomposition  product.  This 
association  with  calcite  suggests  derivation  from  it,  and  the  proximity  in  many 
of  these  cases  of  partly  oxidized  pyrite  indicates  that  the  sulphuric  acid  derived 
from  the  pyrite  has  wrought  the  change.  The  surface  waters  containing  oxygen 
would  decompose  the  pyrite  and  form  limonite  (which  is  found  near  the  surface) 
and  sulphuric  acid.  The  latter  would  decompose  the  calcite  (which  itself  was 
formed  by  hydrothermal  processes  from  the  calcareous  silicates  of  the  andesite) 
and  produce  gypsum  and  carbonic  acid. 

In  the  Fraction  workings,  at  a  depth  of  400  feet  and  in  the  West  End  and 
the  MacNamara  (the  latter  at  280  feet),  fissures  were  tapped  which  contained  a 
heavy  odorless  gas  that  put  out  lights  and  necessitated  temporary  interruption 
'of  work.  This  gas  was  immediately  dispersed  by  the  ventilation,  indicating  that 
the  fissures  were  reservoirs  and  not  outlets.  The  writer  has  not  been  able  to 
collect  any  of  the  gas,  but  in  all  these  cases  it  was  encountered  near  calcareous, 
pyritiferous,  and  gypseous  andesite,  and  it  is  likely  that  it  may  have  been  carbonic 
acid,  the  final  result  of  the  reactions  indicated,  which  accumulated  in  cavities. 

SECONDARY    SULPHIDES. 
PYRARGYRITE,    ARGENTITE,    AND  NATIVE    SILVER. 

Wherever  observed  macroscopically,  pyrargyrite  (ruby  silver)  and  to  a  great 
extent,  also,  argentite  (silver  glance)  coat  crevices  which  cut  the  primary  ore  and 
are  evidently  of  secondary  deposition.  These  minerals  were  found  in  comparative 
abundance  in  the  Fraction,  in  the  unoxidized  ores  on  the  237-  and  300-foot  levels; 
on  the  237-foot  level  native  silver  occurred  also,  coating  cracks,  and  also  plainly 


SECONDARY    SULPHIDES.  95 

secondary.  In  the  Mizpah,  ruby  silver  is  rare,  but  it  has  been  noted  in  the 
250-foot  level,  where,  from  microscopic  examination,  it  appeared  that  both  ruby 
and  horn  silver  are  secondary  to  the  original  black  silver  sulphide.0 

ARGENTITE,    POLYBASITE,    AND   CHALCOPYRITE   IN    DRUSES. 

In  the  Montana  Tonopah,  at  a  depth  of  about  500  feet,  were  found  specimens 
showing  good  crystals  of  argentite,  polybasite  (in  part,  perhaps,  stephanite),  and 
chalcopyrite,  often  sitting  free  in  cracks  and  little  druses  in  the  solid  rich  sulphide 
ore.  Evidently  these  minerals  were  formed  subsequent  to  the  solid  ore,  and  the 
silver  seems  to  have  been  concentrated  from  the  main  mass  and  to  have  been 
precipitated  in  the  crevices.  Secondary  pyrite  has  also  been  noted,  for  example, 
in  the  Fraction  mine,  sitting  free  upon  quartz  crystals  which  line  druses  in  the 
vein. 

COMPARISON    OF   SECONDARY   SULPHIDES   AT   NEIHART   AND   TOSOPAH. 

At  Neihart,  Mont.,  Mr.  W.  H.  Weed*  has  described  polybasite  and  pyrar- 
gyrite  (ruby  silver)  incrusting  impure  galena,  blende,  pyrite,  quartz,  and 
barite.  These  crusts  are  now  forming  in  vugs  and  watercourses  filled  by 
sluggish  descending  surface  waters.  The  polybasite  seems  to  be  an  alteration 
product  of  galena,  and  in  some  cases  pyrargyrite  is  undoubtedly  derived  from  it. 
Blende  is  also  in  some  cases  secondary.  Argentite  is  probably  present.  Mr. 
Weed  explains  the  secondary  precipitation  by  lixiviation  of  the  ores  by  iron 
sulphate,  formed  by  oxidation  of  iron  sulphide  (pyrite). 

The  Tonopah  occurrence  is  analogous,  except  that  here  satisfactory  evidence 
of  the  manner  of  deposition  has  not  been  found.  There  is  little  doubt  that  the 
pyrargyrite  and  argentite  found  along  cracks  were  formed  subsequently  to  and 
are  probably  derived  from  the  primary  ore.  This  primary  ore  is,  however, 
richer  than  that  at  Neihart;  indeed,  it  consists  largely  of  silver  sulphide,  in  part 
antimonial.  For  this  reason  the  mode  of  occurrence  of  polybasite  and  argentite 
in  druses  in  the  rich  Montana  Tonopah  ore  is  not  of  such  plain  import.  In  the 
Montana  Tonopah  it  has  been  shown  that  during  the  period  of  primary  deposition 
the  vein,  after  being  filled,  was  crushed  and  reopened,  and  again  cemented  by 
similar  rich  sulphides,  somewhat  richer  apparently  than  those  of  the  first  deposition 
(see  p.  172);  and  the  polybasite,  argentite,  and  chalcopyrite  in  druses  may  mark 
a  third  and  final  stage  in  the  primary  deposition.  Also,  chalcopyrite  occurs  in 
the  bulk  of  the  ore  as  more  or  less  definite  seams,  apparently  somewhat  later  than 
the  rest,  but  not  clearly  of  different  origin. 

a  Probably  argentite  (see  p.  92).  STrans.  Am.  Inst.  Min.  Eng.,  vol.  30,  p.  434. 


96  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,   NEVADA. 

EVIDENCE  FAVORING   SECONDARY    DEPOSITION    OF   SULPHIDES   BY    DESCENDING  WATERS. 

On  the  other  hand,  the  formation  in  the  oxidized  zone  of  limonite  from  pyrite 
and  of  cerargyrite  from  sulphides  affords  evidence  that  the  metallic  minerals  of 
the  ores  have  actually  been  dissolved  and  reprecipitated  by  surface  waters,  and  in 
several  cases  the  occurrence  of  rub3r  silver  (pyrargyrite)  in  cracks  in  these  partially 
oxidized  ores  shows  beyond  a  doubt  that  it  also  is  due  to  descending  surface 
waters.  Moreover,  some  of  the  ores,  when  studied  microscopically,  show  argentite 
fringing  cerargyrite,  as  if  secondary  to  it.  The  iron  sulphate  necessary  to  the 
solution  of  the  silver  sulphide  has  been  present  (as  is  shown  in  the  alteration  of 
calcite  to  gypsum)  and  the  silver  has  actually  been  dissolved,  and  such  occurrences 
of  secondary  sulphides  as  have  been  described  would  be  the  natural  result.  The 
evidence  therefore  favors  the  view  that  these  secondary  sulphides  in  the  oxidized 
zone  originated  from  descending  surface  waters  and  probably  part,  but  not  all,  of 
the  sulphides  in  druses  in  the  sulphide  ore  have  a  similar  origin. 

The  characteristics  of  the  superficial  alteration  of  the  ores  are  those  which 
naturally  result  from  the  climatic  and  topographic  conditions."  In  all  of  the 
mines  discussed  (yielding  ores)  standing  ground  water  is  lacking;  at  least,  none 
has  been  encountered  up  to  the  considerable  depths  attained  (over  1,100  feet). 
Therefore  the  alteration  is  spotty  and  incomplete,  but  extends  irregularly  to  very 
considerable  depths  in  various  places. 

No  definite  secondary  sulphide  zone  has  been  noted,  the  secondary  sulphides 
being  associated  with  the  predominant  oxides,  chlorides,  etc.,  in  the  oxidized 
zone  and  coating  crevices  in  the  primary  sulphides. 

VEINS  OF  THE  TONOPAH  BHYOLITE-DACITE  PERIOD. 

In  many  mine  workings  there  are  quartz  veins  of  a  certain  class  which  are 
large  and  may  carry  values,  but  which  are  to  be  separated  from  the  principal  ore- 
bearing  system.  These  are  easily  confounded  with  the  veins  of  the  earlier 
andesite,  just  as  the  silicified  Tonopah  rhyolite-dacite,  in  which  they  usually  occur, 
may  be  confounded  with  certain  highly  silicified  phases  of  the  earlier  andesite. 
Such  veins  have  been  encountered  in  the  Belle  of  Tonopah,  the  King  Tonopah, 
the  Mizpah  Extension,  the  Desert  Queen,  North  Star,  Montana  Tonopah,  Mizpah, 
Midway,  MacNamara,  West  End,  Tonopah  Extension,  and  Ohio  Tonopah,  and  are 
described  in  the  detailed  account  of  these  mines.  On  account  of  their  resemblance 
to  the  earlier  andesite  veins  they  have  been  the  object  of  a  good  deal  of  exploration 
and  development  work,  which,  on  the  average,  has  been  decidedly  unprofitable. 

In  connection  with  the  occurrence  of  such  veins,  which  are  described  elsewhere 
in  the  report  in  the  mine  descriptions,  another  occurrence,  somewhat  different 
from  the  rest  and  having  considerable  interest,  majr  be  described. 

"Spurr,  J.  E.,  Geology  Applied  to  Mining,  pp.  275-276. 


VEINS   OF   TONOPAH    RHYOLITE-DACITE.  97 

Just  beyond  the  western  corner  of  the  area  mapped  (PI.  XI),  opposite  Siebert 
Mountain,  a  group  of  three  low  hills  rises  above  the  plain.  One  of  these 
hills  is  capped  by  a  patch  of  dacite,  whose  resistance  to  erosion  has  probably 
caused  the  hill.  The  rest  of  this  hill  and  all  of  the  other  two  are  composed  of 
white  tuff  mixed  with  beds  of  conglomerate,  plainly  referable  to  the  white  tuffs 
of  the  area  mapped.  The  origin  of  the  two  hills,  which  are  composed  entirely  of 
tuffs,  is  due  to  two  elliptical  areas  where  these  tuffs  and  conglomerates  have  been 
thoroughly  silicitied  and  changed  to  a  quartzite-like  condition.  Some  mineral- 
ization has  accompanied  the  silicification.  A  random  specimen  of  the  silicified 
material  from  the  smaller  of  the  two  hills  thus  formed  was  reported  to  the 
writer  to  have  yielded  on  assay  $8  in  gold  and  no  silver.  This  silicitication  and 
mineralization  is  evidently  the  work  of  powerful  hot  springs,  and  the  elliptical 
shape  of  the  silicitied  areas  shows  that  the  springs  rose  along  pipe-like  channels 
and  not  along  definite  fractures.  These  deposits  are  probably  of  practically  the 
same  age  and  origin  as  the  veins  in  the  Tonopah  rhyolite-dacite. 

CHARACTERISTICS  OF  RHYOLITE-DACITE  VEINS. 

The  veins  of  this  period  are  characterized  by  irregularity  and  by  lack  of 
definition  and  persistence,  though  their  size  may  locally  be  great.  As  a  rule  they 
are  elongated  and  have  the  appearance  of  veins,  but  can  not  be  followed  as  far  either 
on  the  strike  or  dip  as  true  veins  may.  They  may  disappear  by  scattering  and 
passing  into  a  silicified  wall  rock,  or  may  be  cut  off  along  a  cross-wall  fracture  in 
the  same  manner  as  some  of  the  veins  in  the  earlier  andesite  described  on  p.  85. 
The  quartz  is  as  a  rule  dense  and  jaspery,  and  is  white,  gray,  or  black;  it  is  therefore 
usually  of  different  appearance  from  the  white  quartz  of  the  earlier  andesite  veins. 
The  veins  are  usually  barren  or  contain  only  very  small  quantities  of  gold  and  silver, 
except  locally,  as  in  the  Desert  Queen,  where  rich  bunches  of  ore  may  occur,  though 
usually  of  limited  and  irregular  extent  (fig.  13).  Like  the  veins  of  the  earlier 
andesite  the  rhyolite-dacite  veins  very  frequently  contain  adularia,  and  in  one  case 
probable  albite  was  noted  (see  p.  197),  a  mineral  which  has  not  been  detected  in  the 
andesite  veins.  In  the  Ohio  Tonopah  barite  has  been  found  as  a  gangue  mineral  with 
the  rhyolite-dacite  veins.  This  mineral  has  not  yet  been  found  in  connection  with 
the  earlier  andesite  mineralization.  In  the  Desert  Queen  and  the  North  Star,  where 
quartz  of  the  rhyolite-dacite  period  has  been  cut  by  drifting,  a  green  stain  forms  on 
the  walls,  which  is  a  basic  copper  sulphate.  This  phenomenon  has  not  yet  been 
noted  in  connection  with  the  earlier  andesite  mineralization.  A  characteristic  of  the 
rhyolite-dacite  veins,  to  which  there  are,  however,  numerous  exceptions,  is  the 
1684S— No.  42—05 7 


98 


GEOLOGY    OF   TONOPAH   MINING    DISTRICT,  NEVADA. 


300 


4-oofeet 


Fie.  13.— Rbyolltlc  veins  (latur  period)  in  Tonopah  rh yolite-daclte,  814-foot  level,  Desert  Queen  shaft,  showing  irregularity 

and  lack  of  persistence.    Horizontal  plan. 


VEINS    OF    TONOPAH    RHYOLITE-DACITE.  99 

greater  ratio  of  gold  to  silver  in  them  as  compared  to  that  in  the  earlier  andesite 
veins.  In  the  earlier  andesite  veins  the  gold  averages  about  two-fifths  of  the  value, 
the  silver  three-fifths,  while  in  the  rhyolite-dacite  veins  the  gold  is  likely  to  exceed 
this  amount  and  sometimes  occurs  with  practically  no  silver,  although  the  proportion 
is  very  changeable.  Very  often  again  the  proportion  of  gold  and  silver  is  the  same 
as  in  the  earlier  andesite  veins. 

AGE  OF  TONOPAH  RHYOLITE-DACITE  VEINS. 

These  veins  are  younger  than  the  Tonopah  rhyolite-dacite,  in  which  they 
usually  occur.  In  the  mine  workings  referred  to  above  this  lava  is  a  deep-seated 
injection  corresponding  in  age  and  composition  to  a  great  mass  of  surface  breccias 
and  tuffs  in  the  southern  half  of  the  area  mapped.  Even  in  the  lower  part  of  the 
white  tuffs  or  lake  beds  which  succeeded  the  deposition  of  the  volcanic  ejectamenta 
of  this  period  there  are  intrusive  sheets  of  the  rhyolite-dacite.  In  this  portion 
of  the  tuffs  occur  the  elliptical  outcrops  of  the  pipe-like  deposits,  formed  by  hot 
springs  in  the  hills  west  of  Siebert  Mountain.  Thus  the  period  of  this  mineral- 
ization was,  in  broad  terms,  contemporaneous  with  the  volcanic  activity  of  the 
Tonopah  rhyolite-dacite  period,  and  very  likely  persisted  for  some  time  after- 
wards. These  veins  are  plainly  the  results  of  ascending  hot  waters,  and  represent 
the  effects  of  the  Tonopah  rhyolite-dacite  eruption.  They  have  the  same  relation 
to  these  eruptions  that  the  earlier  andesite  veins  had  to  the  eruptions  of  the 
earlier  andesite. 

The  characteristic  lack  of  definition  and  persistence  in  these  veins  as  compared 
with  the  veins  in  the  earlier  andesite  shows  that  at  the  time  they  were  formed  no 
definite  fracture  zones  were  available  as  channels,  so  that  the  ascending  waters  had 
to  force  themselves  up  along  irregular  courses.  This  means  that  the  faulting  now 
so  characteristic  of  the  district  had  not  begun  at  the  time  of  this  mineralization, 
and  therefore  that  this  mineralization  ceased  before  the  beginning  of  that  period 
of  rhyolite  and  dacite  injections  and  eruptions  which  is  marked  by  the  rhyolite 
and  dacite  necks  that  form  the  hills  around  Tonopah.  The  mineralization  is  then 
probably  the  same  in  time,  nature,  and  origin  as  that  at  Gold  Mountain,  4  miles 
south  of  Tonopah,"  and  very  likely  similar  to  that  in  the  newly  discovered  camp 
of  Goldfields,  about  28  miles  south  of  Tonopah. 

GENERAL    RESTRICTION    OF   VEINS    TO    RHYOLITE-DACITE. 

At  first  it  seems  strange  that  in  underground  workings  like  the  West  End* 
the  MacNamara,  etc.,  these  rhyolite-dacite  veins  do  not  extend  into  the  earlier 
andesite  in  which  the  rhyolite-dacite  is  intrusive.  The  fact  that  such  veins 

a  Bull.  U.  S.  Geol.  Survey  No.  218,  p.  87. 


100  GEOLOGY    OB'    TONOPAH    MINING    DISTRICT,   NEVADA. 

extend  to  the  contact  of  the  andesite  and  do  not  enter  it,  raises  at  first  a  doubt 
as  to  whether  the  andesite  is  not  really  the  younger  rock  instead  of  the  older. 
In  some  of  the  shafts  mentioned  the  andesite  is  soft  and  very  little  silicified, 
while  the  amount  of  silicification  in  the  rhyolite-dacite  is  very  great.  However, 
there  is  no  doubt  of  the  relative  age  of  the  rhyolite-dacite  as  given  on  p.  43, 
and  the  reason  for  the  described  phenomenon  appears  upon  reflection.  The 
rhyolite-dacite  consists  mainly  of  volcanic  glass  and  was  injected  into  the  earlier 
andesite  after  this  was  thoroughly  decomposed  and  softened  as  the  result  of  the 
action  of  hot  spring  waters  that  accompanied  and  caused  the  principal  minerali- 
zation. Any  slight  subsequent  strains  in  the  earth  resulting  from  volcanic  action 
shattered  this  fresh  and  glassy  rock,  but  formed  no  fractures  or  fissures  in  the 
soft  adjacent  andesite.  The  hot  waters  that  rose  immediately  after  the  rhyolite- 
dacite  eruptions  found  almost  their  only  channels  in  the  fractured  and  fissured 
glassy  rock  to  which  they  owed  their  origin.  Therefore  the  veins  that  they 
formed  are  confined  chiefly  to  this  rock.  Evidence  of  the  correctness  of  this 
explanation  is  furnished  by  the  thick  veins  of  this  period  that  are  found  on  the 
contact  of  the  rhyolite-dacite  sheet  with  the  overlying  decomposed  andesite.  Such 
veins  are  often  found  at  this  place  and  the  accompanying  silicification  is  very 
great,  but  is  almost  invariably  confined  to  the  rhyolite-dacite  near  the  contact. 
Such,  for  example,  is  the  situation  in  the  Mizpah  Extension,  the  MacNamara, 
Tonopah  Extension,  and  West  End,  and  to  a  less  degree  in  the  Ohio  Tonopah. 
These  things  show  that  the  ascending  hot  waters,  circulating  through  the  fractured 
rhyolite-dacite,  rose  until  at  the  contact  with  the  overlying  soft  andesite  they 
found  a  practically  impervious  barrier,  along  whose  lower  contact  they  circulated 
and  deposited  the  materials  which  they  held  in  solution. 

Subsequent  to  this  formation  of  quartz  veins  and  attendant  silicification, 
similar  differences  between  the  rhyolite-dacite  and  the  andesite  with  reference  to 
brittleness  continued,  so  at  the  present  day  the  silicified  rhyolite-dacite  is  found 
to  be  extremely  faulted  and  fractured  and  to  contain  open  fissures,  features  which 
are  not  present  to  the  same  extent  in  the  adjacent  andesite. 

EFFECT  OF  WATERS  PRODUCING  THE  TONOPAH   RHYOLITE-DACITE  VEINS  ON 

EARLIER  FORMED  VEINS. 

Although  as  a  rule  decomposed  andesite  seems  to  have  presented  a  formidable 
barrier  to  the  circulating  waters  accompanying  the  Tonopah  rhyolite-dacite,  in 
some  places  the  waters  must  have  traversed  the  andesite  and  found  their  way 
along  the  andesitic  veins.  Indeed,  it  is  along  these  brittle  veins  and  the  brittle 
silicified  adjacent  andesite  that  fractures  and  fissures  must  have  been  most  easily 
formed  at  this  period.  In  the  case  of  the  Tonopah  Extension,  as  described 


CALCITIC    VEINS    OF    ARARAT    MOUNTAIN.  101 

elsewhere  (see  p.  182),  the  earlier  andesite  vein  has  been  reopened  and  a  new  vein 
of  barren  jaspery  quartz  formed  along  the  hanging  wall.  This  is  probably  due 
to  waters  of  the  rhyolite-dacite  period  of  mineralization.  In  the  case  just  men- 
tioned the  new  quartz  is  barren  as  compared  with  the  old.  It  is  evident,  however, 
that  such  solutions  must  have  dissolved  a  great  deal  of  the  gold  and  silver  contained 
in  the  earlier  veins,  and  naturally  may  have  reprecipitated  it  elsewhere.  In  this 
case  the  ores  might  be  reprecipitated  in  a  concentrated  form.  This  very  likely 
has  been  the  case  in  the  Montana  Tonopah,  where,  as  described  (see  p.  171),  the 
original  vein  has  been  reopened  and  in  the  fissure  thus  formed  minerals  similar  to 
those  in  the  older  vein,  but  richer  in  gold  and  silver,  have  been  precipitated  in 
crustified  form.  It  is  very  likely  that  this  was  the  work  of  the  waters  of  the 
rhyolite-dacite  period,  of  the  same  kind  and  character  as  those  to  which  the 
barren  quartz  hanging-wall  portion  of  the  vein  in  the  Tonopah  Extension  is 
due. 

Again,  it  is  natural  that  such  waters  may  have  dissolved  some  of  the  metallic 
contents  of  the  older  veins  and,  instead  of  precipitating  them  within  these  veins, 
may  have  carried  them  out  and  deposited  them  elsewhere,  as,  for  example,  in  the 
veins  of  the  rliyolite-dacite,  forming  bunches  of  high-grade  ore  in  these  usually 
barren  veins.  This  may  be  the  explanation  of  the  comparatively  small  amount 
of  rich  ore  found  in  the  rhyolite-dacite  veins,  as,  for  instance,  in  the  Desert  Queen 
and  the  MacNamara.  These  are  practically  the  only  cases  of  high-grade  ore  in 
the  district  in  veins  of  this  period,  and  in  both  cases  the  veins  are  in  the  vicinity 
of  rich  earlier  andesite  veins  and  the  ores  have  a  character  altogether  similar  to 
that  of  the  earlier  veins.  Outside  of  the  earlier  andesite  vein  region,  the  veins 
in  the  rhyolite-dacite  have  been  found  to  be  frequently  large,  but  typically  are 
low  grade  or  barren. 

THE  CALCITIC  VKINS  OF  ARARAT  MOUNTAIN. 

THE  RHYOLITE  OF  ARARAT  A  VOLCANIC   PLUG. 

The  top  of  Ararat  Mountain  is  composed  of  white  rhyolite  like  that  of  Mount 
Oddie.  On  the  southwest  side  this  is  intrusive  into  the  later  andesite,  and  on 
the  other  sides  into  the  glassy  Tonopah  rhyolite-dacite,  which  is  itself  intrusive 
into  the  later  andesite.  The  area  of  white  rhyolite  is  broadly  ellipical  in  outline, 
with  the  longer  axis  of  the  ellipse,  as  in  the  case  of  most  of  the  other  hills  on 
the  map,  lying  in  a  general  east-west  direction  (PI.  XIV). 

The  contact,  as  is  shown  by  the  Wingfield  tunnel  and  the  Boston  Tonopah 
shaft  and  in  other  places,  pitches  steeply  all  around.  The  rhyolite  is  then  in  the 
nature  of  a  volcanic  column  or  plug  which  has  been  forced  up  into  the  older 
rocks,  and  which  probably  occupied  the  vent  of  an  old  volcano,  now  removed  by 
erosion. 


102 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 


FLOW  BRECCIATION   NEAR  CONTACT. 


Near  the  contact  in  many  places  the  rhyolite  is  peculiarly  brecciated,  showing 
great  blocks  jumbled  together,  with,  however,  rhyolitic  matrix  between.  The  dim 
outline  of  these  blocks  and  the  rhyolitic  matrix  show  that  .they  were  formed  when 
the  lava  was  in  the  process  of  cooling  and  only  partly  rigid.  This  brecciation 
decreases  away  from  the  contact,  but  in  places  occupies  a  zone  upwards  of  100 
feet  wide.  The  breccia  indicates  that  the  plug  was  forced  upward  while  cooling. 

FISSURE  VEINS  IN  THE  RHYOLITE  PLUG. 

Many  sharp  fractures  cut  the  rhyolite,  increasing  in  number  as  the  contact 
is  reached.  These  are  chiefly  parallel  to  the  contact.  They  have  been  filled  with 


paraiiei  10  me  contact,     iney  nave  oeen  nilea 

m^mwsm^--^-^ 

m$mit:mmm.^'\  •::;'••>>:• 

Rfe^Kili^:^ 


^pfStlla^'^'^f; 
liliiSSfM^'-^^^ 

fif-r:i-51-:r-:-.--"H -•,!--  V-J---L-----"-:L--«  "  ,  •*  ,  -  '.  -  .-'-'. 


P^lffi-^^W^ 

faiii^ft  >3i  AsV-4te 


Z  32 

e» i  _ 


5  feet 


FIG.  14. — Cross  section  of  outcropping  fissure  vein  In  Ararat  rhyolite  neek  near  margin.  Heptile  claim,  north  of  the 
Boston  Tonopah  shaft.  1,  Dark-brown  calcite  and  siderite,  mixed;  2,  white  calcite,  beautifully  banded;  3,  quartz 
mixed  with  calcite;  4,  white  rhyolite  (wall  rook). 

material  as  described  below,  and  constitute  veins  that  are  locally  as  much  as  20  feet 
thick,  but  are  exceedingly  irregular  and  nonpersistent.  These  veins  conspicuously 
follow  the  contact  and  are  coterminous  with  it;  they  do  not  extend  into  the  older 
intruded  rocks,  but  often  run  back  into  the  rhyolite.  A  prominent  line  of  veins, 
as  shown  in  PI.  XIV,  extends  due  north  across  the  top  of  the  hill,  from  the  vicinity 
of  the  Wingfield  tunnel.  These  are  fine  examples  of  veins  which  have  filled  open 
fissures. 


U   S    GEOLOGICAL    SURVEY 


PROFESSIONAL    PAPER    NO    4.2     PL    XIV 


MAP  SHOWING  THE  CHIEF  VEINS  OF  ARARAT  MOUNTAIN  AND  THEIR 
RESTRICTION  TO  THE  ODD  IE  RHYOLITE]  PLUG 

By  >J.  E.Spurr 


IHO'i- 
S  oale 


isoo  Feel 


Odcfir  rtivitliip  shtt *vr>  in  yrevti .  veins  in  red . 


CALOITIC    VEINS    OF    ARARAT    MOUNTAIN. 


103 


On  the  Reptile  claim,  above  the  Wingfield  tunnel,  an  outcropping  vein  of  this 
material  is  beautifully  banded,  and  consists  of  brown  and  white  calcite  and  some 
quartz  (fig.  1-t)."  Some  assays  of  this  are  said  to  show  a  value  as  high  as  $20,  all 
in  gold,  but  it  is  mostly  barren.  Several  small  veins  near  by  are  of  the  same 
character.  One  of  these  distinctly  shows  quartz  as  a  later  deposit  than  calcite 
(fig.  15).  These  veins  have  a  general  northerly  trend,  and  the  vein  zone  can  be 
followed  all  the  way  across  the  hill  to  the  contact  on  the  north,  but  no  farther. 
Each  vein  can  be  followed  only  a  short  distance,  however,  when  it  becomes  con- 
fused by  reason  of  splitting,  straggling,  and  thinning,  while  a  lateral  vein  may 
thicken  up  so  as  to  become  of  predominating  importance. 

At  the  contact  between  the  white  rhyolite  plug  and  the  glassy  Tonopah  rhyolite- 
dacite,  on  the  east  side, 
an  8-inch  vein  of  banded 
white  and  brown  calcite 
and  siderite,  cementing 
a  fissure  in  the  white 
rhyolite,  was  observed. 
This  has  a  strike  of  N. 
10°  W.  and  a  dip  of  70° 
to  the  east. 

On  the  oppo.site  or 
west  side  of  the  intru- 
sive plug,  near  or  at  the 
contact  between  it  and 
the  later  andesite,  there 
is  a  vein  of  beautifully 
crustified  crystalline  cal- 
cite, locally  20  feet 
thick.  The  rhyolite  on 
one  side  of  the  vein  has  been  silicified  so  as  to  form  a  pale-yellow  jasper. 

It  will  be  noted  from  PI.  XIV  that  these  veins,  although  their  position  and 
trend  are  governed  to  a  large  extent  by  the  contact,  have  a  general  north-south 
trend  independent  of  it.  This  indicates  that  the  chief  strain  at  the  time  the  fissures 
were  formed  was  in  a  direction  nearly  at  right  angles  to  the  longest  axis  of  the 
elliptical  horizontal  cross  section  of  the  volcanic  plug.  This  north-south  trend  is  at 
right  angles  to  the  principal  trend  of  the  ore-bearing  veins  in  the  earlier  andesite, 
formed  at  an  earlier  epoch  (fig.  12,  p.  84). 

a  Dr.  W.  F.  Hillebrand  kindly  examined  the  dark-colored  carbonate  for  the  writer.  He  finds  it  essentially  calcite, 
with  very  small  amounts  of  iron  and  manganese  carbonates,  a  considerable  amount  of  mechanically  included  hematite, 
and  some  black  manganese  oxide. 


-iz$< 


3  feet 


FIG.  15.— Vertical  cross  section  oi  outcropping  tissure  vein,  20  feet  west  of  section 
shown  in  fig.   14.    1.  Calcite  with  angular  rhyolite  fragments;  2,  quartz;  3, 

white  rhyolite. 


104  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

FISSURES    DUE   TO   MOVEMENT   AFTER   CONSOLIDATION. 

These  fissures  and  fractures,  judging  from  their  distribution  and  direction, 
plainly  resulted  from  the  continuation  of  the  driving  upward  of  the  plug  after  con- 
solidation was  practically  complete. 

The  movement  thus  indicated  is  like  that  which  was  manifested  by  the  plug  of 
Mont  Pele'e  in  Martinique  subsequent  to  the  late  eruptions,  when  it  was  forced 
upward  after  solidification,  so  as  to  tower  several  hundred  feet  in  the  air.0  Around 
the  base  of  such  a  plug  as  Pelee's,  phenomena  like  those  on  Ararat  must  have  taken 
place. 

The  fillings  are  evidently  the  result  of  ascending  hot  water  which  followed  the 
channels  thus  opened  and  cemented  them.  That  such  large  open  spaces  due  to 
rending  could  have  been  formed  indicates  that  the  spot  was  not  very  far  distant 
from  the  surface. 

PARAGENESIS   OF   VEIN   MATERIALS. 

The  substances  deposited  in  the  openings  also  are  simple,  as  compared  with 
those  of  other  periods  of  vein  formation  in  the  district.  The  alteration  of  the 
rhyolite  is  confined  to  silicification  and  slight  bleaching  of  the  biotite.  Some 
of  the  specimens  from  the  Wingfield  tunnel  show  feldspar  phenocrysts  completely 
altered  to  microcrystalline  and  cryptociystalline  silica.  In  many  cases  this  silici- 
fication seems  to  have  preceded  the  deposition  of  the  carbonates,  for  the  latter 
are  deposited  in  cavities  upon  the  silicified  rhyolite.  In  other  cases,  however, 
the  jaspery  and  chalcedonic  quartz,  which  is  often  part  of  the  fissure  filling,  is 
plainly  later  than  the  carbonates.  In  several  cases  white  calcite  was  observed 
to  be  later  than  the  dark  or  ferruginous  calcite  in  origin. 

COMPOSITION   OF   VEIN-FORMING   WATERS. 

No  sericite  was  observed  to  be  developed  in  the  wall  rocks,  hence  it  seems 
probable  that  the  waters  did  not  contain  fluorine  (see  p.  232),  or  that  their  temperature 
was  very  moderate,  or  both.  Indeed,  they  do  not  give  evidence  of  having  contained 
anything  beyond  silica,  lime,  iron,  and  manganese  carbonates.  Their  content  of 
gold  was  small,  for  the  veins  are  generally  practically  barren.  No  larger  amount 
of  this  metal  is  likely  to  have  been  present  than  has  been  detected  in  many  hot 
springs  issuing  at  the  surface.  The  presence  of  iron  is  contrasted  with  the 
probable  absence  of  iron  in  the  solutions  which  produced  the  earlier  andesite. 

oHovey,  E.  O..  Am.  Jour.  Sci.,  4th  ser.,  vol.   16,  pp.  269-281.    Russell,  I.  C.,  Science,  vol.  17,   pp.  792-796;  Am.  Jour. 
8cl.,  4th  ser.,  vol.  17, 1904. 


CHAPTER    HI. 

PRESENT  SUBTERRANEAN  WATER, 

WATER  ENCOUNTERED   IN   MINING   OPERATIONS. 

The  Desert  Queen  shaft  is  1,114  feet  deep.  It  is  perfectly  dry,  except  at 
the  contact  of  the  rhyolite  and  later  andesite  at  a  depth  of  a  little  over  300  feet, 
where  water  following  the  contact  zone  was  encountered.  Along  the  watercourse, 
which  strikes  north  and  south  and  dips  60°  east,  the  rocks  have  been  altered  to 
clay.  Fragments  of  rocks  in  the  channel  show  fresh  pyrite  on  cracks,  indicating 
that  these  waters  have  deposited  the  sulphide.  The  water  tasted  very  slightly 
astringent;  when  first  encountered  it  was  tepid,  but  afterwards  it  became  cool. 

The  water  was  encountered  in  October,  1902,  when  the  flow  was  about  3,000 
gallons  per  twenty-four  hours;  it  gradually  diminished,  till  in  six  weeks  it  was 
only  250  gallons,  and  later  in  the  fall  shrunk  to  100  gallons.  In  the  spring, 
however,  the  flow  increased  to  250  gallons,  and  the  water  was  cold. 

These  data  show  that  the  water  of  the  contact  zone  was  contained  in  a 
comparatively  small  basin  or  reservoir,  whose  surface  was  quickly  lowered,  and 
the  increase  in  the  spring  with  the  melting  snow  indicates  that  this  basin  is  fed 
from  the  surface. 

The  Halifax  shaft  encountered  water  below  600  feet;  at  640  feet  the  flow, 
on  July  17,  1903,  was  estimated  by  the  manager  at  12,000  to  15,000  gallons  a 
day,  and  on  July  20  at  20,000  to  30,000,  so  it  was  necessary  to  stop  work  pending 
the  arrival  of  a  pump. 

A  similar  copious  flow  was  encountered  in  the  Rescue,  situated  just  south  of 
Mizpah  Hill.  At  a  depth  of  250  feet  an  estimated  flow  of  6,000  to  7,000  gallons 
a  day  was  encountered  along  a  crevice  in  the  rhyolite,  striking  northeast  and 
dipping  southwest  at  an  angle  of  about  40°.  Below  this  there  was  no  water  till 
a  depth  of  300  feet  was  reached,  at  which  depth  more  water  came  in  along 
fractures  striking  northwest  and  dipping  northeast.  When  this  second  water  zone 
was  struck  the  supply  of  water  in  the  first  was  reduced,  showing  that  the  two 
zones  are  connected.  On  July  10,  1903,  the  combined  flow  from  the  two  was 
about  8,000  gallons;  on  July  17  it  was  estimated  by  the  manager  to  be  from 
25,000  to  30,000  gallons. 

The  Gold  Hill  shaft  was  dry  to  the  bottom  (490  feet),  but  a  drift  running 
northward  from  the  bottom  struck  water  in  fractures  a  short  distance  from  the 

105 


106  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 

shaft.  The  south  drift  was  dry.  The  water  here  was  estimated  at  one  time  to  be 
7,000  or  8,000  gallons  a  day. 

The  Belle  of  Tonopah  shaft  encountered  water  along  fractures  at  a  depth  of 
150  feet.  This  was  drained,  and  another  water  seam  was  cut  at  190  feet.  The 
rock  is  soft  later  andesite,  very  full  of  pyrite,  indicating,  as  at  the  Desert  Queen 
shaft,  that  these  waters  deposit  pyrite. 

The  Golden  Anchor  struck  water  at  a  depth  of  130  feet  and  also  farther 
down  along  fractures.  One  fracture  from  which  water  issued,  seen  by  the  writer 
at  200  feet,  was  perpendicular,  and  had  a  course  of  N.  70°  W.  This  fracture 
had  been  cemented  by  calcite  and  reopened.  The  Silver  Top,  east  of  the  Golden 
Anchor,  encountered  water  at  a  depth  of  180  feet. 

The  Mizpah  Extension  encountered  water  at  a  depth  of  430  feet  at  the  contact 
of  Oddie  rhyolite  and  Tonopah  rhyolite-dacite.  The  water  runs  on  top  of  14  feet 
of  wet  clay,  formed  by  rock  decomposition.  The  water  zone  strikes  N.  30°  W. 
and  dip  northeast  at  an  angle  of  40°.  At  the  time  of  the  writer's  inquiry,  in 
November,  1902,  the  flow  was  about  300  gallons  a  day.  The  shaft  was  sunk  to  a 
depth  of  800  feet  without  encountering  any  more  water. 

The  other  shafts  in  the  district  were  quite  dry  at  the  time  the  writer  made 
his  observations.  Their  depths  at  that  time  or  soon  afterwards  were  as  follows: 

Depths  of  dry  shafts  in  Tonopah  district. 

Feet. 

King  Tonopah 300 

Boston  Tonopah 300 

Behnont 340 

North  Star« 1,050 

Siebert 938 

Valley  View 700 

Stone  Cabin 400 

Molly 468 

Montana  Tonopah 765 

Midway 635 

Tonopah  Extension 485 

MacNarnara 500 

West  End 780 

Fraction 400 

Wandering  Boy 500 

Tonopah  and  California 650 

Tonopah  City 500 

Ohio  Tonopah 756 

BigTono 300 

Fraction  Extension 300 

New  York  Tonopah 745 


a  A  little  seepage  along  a  fault  zone  at  a  depth  of  720  feet. 


GROUND    WATER.  107 

OUTCROPPING  WATER  ZONES. 

Previous  to  the  discovery  of  the  water  in  some  of  the  shafts  described  the  entire 
water  supply  of  the  town  of  Tonopah  was  obtained  from  wells  4  miles  to  the 
north,  where  geologic  and  topographic  conditions  are  similar  to  those  at  Tonopah. 
Here,  in  a  distance  of  a  half  mile  or  more,  along  a  small  east-west  valley,  are  a 
number  of  wells,  most  of  which  reach  water  within  30  to  40  feet  of  the  surface. 
The  wells  are  in  solid  later  andesite,  and  the  water  circulates  along  a  fractured 
(probably  faulted)  zone.  The  trend  of  the  water  zone  corresponds  with  that  of 
the  valley,  which  has  probably  been  eroded  along  this  belt  of  fractures. 

These  water  zones  can  often  be  recognized  at  the  surface  by  the  presence  of 
taller  and  greener  vegetation  or  by  plants  requiring  so  much  water  that  they 
would  not  thrive  under  the  usual  arid  conditions. 

DISTRIBUTION  AND  EXPLANATION  OF  WATER  ZONES. 

The  above  data  show  that  while  some  of  the  Tonopah  shafts  have  reached 
depths  of  over  1,000  feet  (in  the  case  of  the  Desert  Queen  over  1,100)  no  general 
body  of  ground  water  has  been  encountered,  though  the  rocks  are  extremely 
fractured;  yet  along  certain  steeply  inclined  fracture  zones  water  is  found 
sometimes  quite  near  the  surface  and  occasionally  in  considerable  quantity.  This 
water  is  cool,  is  sufficiently  nonmineral  to  be  fair  drinking  water,  and  is 
undoubtedly  the  storage  of  precipitation. 

These  water  zones  appear  to  be  widely  spaced.  They  have  been  noted  only 
in  rigid  and  brittle  rock — rhyolite  and  andesite.  They  seem  to  occur  especially 
along  intrusive  contacts,  where  one  rock  has  been  shattered  by  the  intrusion  of 
another.  They  are  often,  perhaps  usually,  accompanied  by  a  clayey  state  of  the 
decomposed  rock.  This  decomposed  rock,  while  itself  undoubtedly  due  to  the 
waters,  now  forms  an  impervious  bottom  or  foot  wall  of  the  fractured  zone  and 
keeps  these  waters  from  penetrating  the  underlying  dry  and  fractured  rocks. 
Thus  the  water  channel  or  basin  has  a  dike-like  shape.  It  appears  probable  that 
similar  clays  may  limit  these  water  basins  in  depth,  limiting  the  downward  extent 
of  the  zone-shaped  basins,  and  thus  explain  why  they  are  found  sometimes  so  near 
the  surface  in  a  region  apparently  without  universal  ground  water. 

USUAL  ABSORPTION  OF  PRECIPITATION  BY  ROCKS. 

In  the  southern  half  of  the  area  shown  on  the  Tonopah  map  (PI.  XI),  in  the 
depressed  area  capped  by  volcanic  breccias,  no  water  has  been  encountered,  even  in 
shafts  over  700  feet  deep,  although  some  shafts,  as  the  Ohio  Tonopah  for  instance, 
have  passed  through  the  soft  breccia  to  a  rigid  and  fractured  rock  below. 
Furthermore,  in  the  breccia-covered  region  to  the  south,  the  writer  does  not 


108  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,   NEVADA. 

know  of  any  water  or  water  signs,  while  to  the  north,  in  the  hard  rock,  water 
zones  outcrop  in  various  places,  both  on  and  beyond  the  area  mapped.  The 
explanation  of  this  is  probably  that  the  porous  breccias  and  tuffs  absorb  the 
scanty  precipitation  like  a  sponge. 

Even  where  rigid  fractured  rocks  outcrop,  the  scanty  descending  water 
normally  sinks  as  through  a  sieve,  using  itself  up  in  kaolinization,  the  formation 
of  limonite,  and  other  hydration  processes,  and  moistening  the  dry  rocks  with 
interstitial  water.  Fresh  rock  taken  from  the  Fraction  and  other  shafts  in  frosty 
weather  was  observed  by  the  writer  to  steam  vigorously  in  the  cold  air,  though 
the  mines  are  perfectly  dry.  It  is  doubtful  if  there  is  enough  of  this  water  left 
to  form  a  standing  body  of  ground  water  at  any  depth.  Where,  however, 
kaolinization  and  other  processes  have  formed  clay  seams,  the  water  ma}"  be 
detained  and  even  stored  at  any  depth  from  the  surface  downward;  and  other 
impervious  rock  materials  may  operate  in  the  same  way. 


CHAPTER   IV. 

PHYSIOGRAPHY. 
ORIGIN"  OF  THE  RANGE  OF  HILIiS. 

The  area  of  the  Tonopah  map  has  been,  from  the  dawn  of  its  available 
record  in  the  middle  Tertiary  down  to  the  present  day.  essentially  a  land  surface, 
save  during  the  period  when  the  white  lake  beds  were  deposited.  At  present 
the  region  consists  of  isolated  buttes  (which  are  usually  denuded  volcanic  necks), 
and  intervening  depressions.  These  buttes  are  irregularly  grouped,  but  occupy  in 
general  a  definite  north-south  belt,  although  this  belt  can  not  be  distinguished  upon 
the  small  detailed  map  which  accompanies  this  report.  The  belt  becomes  higher 
on  the  north,  where  it  is  known  as  the  San  Antonio  Range,  and  rather  lower 
toward  the  south,  where  it  gradually  loses  its  individuality.  The  character  of  the 
rocks  throughout  is  volcanic,  and  evidently  a  large  part  of  the  topographic  relief 
is  due  to  the  fact  that  this  has  been  a  chain  of  Tertiary  volcanoes. 

SKETCH  OF  TERTIARY  AND  QUATERNARY  EROSION. 

GENERAL   FEATURES. 

The  Tonopah  district,  as  limited  by  the  mapped  area,  is  in  the  central  part 
of  this  north-south  topographic  ridge.  The  surface  run-off  drains  mostly  to  the 
west,  but  in  the  eastern  corner  of  the  area  mapped  the  slopes  indicate  that  the 
drainage  is  eastward.  On  both  sides  of  this  volcanic  range  are  broad,  flat,  desert 
valleys.  On  the  west,  which  is  reached  by  a  moderate  and  regular  though  decided 
slope  down  from  Tonopah,  is  the  east  branch  of  Great  Smoky  Valley,  and  on  the 
east  lies  Ralston  Valley.  These  general  topographic  conditions  must  have  existed 
during  most  of  the  period  embracing  the  volcanic  history  of  the  region.  Erosion 
was  steadily  at  work  attacking  the  uplifted  and  outpoured  rocks  of  the  range, 
and  transferring  them  to  the  deep  flanking  valleys;  and  since  much  of  the  volcanic 
material  was  loosely  consolidated  it  must  have  been  transported  with  extraordinary 
rapidity,  especially  as  periods  of  greater  humidity  than  the  present  alternated  with 
the  arid  periods."  Since  the  region  was  probably  all  this  time  without  any  outlet 
to  the  sea,  enormous  amounts  of  detritus  accumulated  in  the  valleys,  partly 

"Spurr,  J.  E.,  Bull.  Geol.  Soc.  Am.,  vol.  12,  p.  250. 

109 


110  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 

filling  up  these  originally  profound  depressions.  This  process  has  continued  up 
to  the  present  day,  and  is  still  going  on,  until  the  volcanic  range  in  which  Tono- 
pah  lies,  like  other  ranges  in  the  district,  is  flanked  on  both  sides  by  nearlv  level 
stretches  of  waste — veritable  waste  lakes — which  constantly  rise  as  the  degradation 
of  the  mountains  progresses.  These  waste  lakes  (kept  level  chiefly  by  the  terrific 
winds  that  travel  up  and  down  between  the  mountain  ranges,  sweeping  the  fine 
material,  unbound  by  moisture  or  by  vegetation,  before  them)  invade  the  deeper 
mountain  valleys  and  overflow  the  lower  hills.  Their  surface  portion  consists  of 
Pleistocene  subaerial  accumulations,  and  it  has  been  unwarrantably  assumed  that 
this,  material  has  a  depth  of  thousands  of  feet,  but  observations  by  the  writer  in  the 
western  part  of  the  State  lead  to  the  conclusion  that  in  many,  perhaps  most,  cases 
the  Pleistocene  cover  is  only  a  veneer,  beneath  which  lie  Tertiary  accumulations." 

MEASURES    FOR   THE   AMOUNT   OF   MATERIAL   ERODED. 

Under  the  conditions  sketched  above  a  large  amount  of  material  must  have  been 
stripped  from  the  area  of  the  Tonopah  quadrangle  and  carried  to  the  valleys. 
Study  of  the  local  geology  affords  us  more  detailed  data  for  this  conclusion.  The 
thick  volcanic  agglomerates  (chiefly  dacitic),  which  occupy  a  large  part  of  the 
southern  half  of  the  area  mapped  and  are  probably  upward  of  a  thousand  feet 
thick,  are  not  represented  in  the  northern  half.  It  is  true  that  these  are  local 
accumulations  and  may  be  essentially  the  remnants  of  bomb  and  cinder  cones  of 
the  earlier  dacitic  eruptions  which  occurred  in  the  southern  and  not  in  the 
northern  region.  Still,  such  material  must  also  have  fallen  over  the  northern 
half  of  the  area  mapped,  even  if  the  quantity  was  smaller;  and  it  is  only  about 
three-quarters  of  a  mile  from  the  New  York  Tonopah  shaft,  where  nearly  800 
feet  of  the  dacite  breccia  has  been  passed  through  and  the  bottom  not  reached, 
to  the  region  east  of  Mizpah  Hill,  where  the  dacite  breccia  is  missing.  This 
disappearance  must  be  due  to  erosion,  which,  moreover,  was  accomplished  before 
or  during  the  deposition  of  the  lake  beds  (Siebert  tuffs),  for  these  in  places  south 
and  east  of  Mount  Oddie  lie  directly  upon  the  earlier  andesitic  rocks. 

That  part  of  the  erosive  work  accomplished  since  the  last  important  geologic 
occurrences — the  intrusion  of  the  volcanic  necks  and  the  faulting — or  since  about 
the  beginning  of  the  Pliocene  (see  pp.  69-70)  can  be  estimated  in  a  more  detailed 
way,  since  the  evidences  are  not  obscured  by  subsequent  events.  The  volcanic  necks 
are  much  modified  by  erosion,  and  on  the  higher  ones,  as  on  Butler  Mountain, 
lateral  drainage  has  pushed  back  and  formed  sharp  dividing  ridges.  It  is  hard  to  say 
how  much  the  solid  lava  columns  have  been  lowered,  but  the  cinder  and  agglomerate 
cones  which  once  surrounded  them  have  been  swept  away  and  only  vestiges  of  them 

aSpurr,  J.  E.,  Bull.  U.  8.  Oeol.  Survey,  No.  208,  pp.  139-140. 


EROSION    IN    ARID    CLIMATES.  Ill 

remain  (p.  45).  These  outer  cones  must  have  been  very  extensive  in  comparison 
with  the  necks  and  must  have  covered  the  whole  area  of  the  quadrangle  deeply. 
There  is  a  difference  of  700  feet  in  elevation  from  the  top  of  Butler  Mountain  to 
the  lowest  point,  near  its  base,  where  the  dacite  neck  cuts  the  intruded  rock,  so 
that  700  feet  is  less  than  the  minimum  possible  thickness  of  the  material  that  has 
been  removed  between  these  two  points. 

A  still  better  measure  of  the  amount  of  erosion  is  supplied  by  a  study  of  the 
faulting.  In  general,  the  southern  half  of  the  area  has  been  depressed  by  faulting 
below  the  northern  half  by  a  distance  which  has  not  been  closely  measured,  but 
which  is  certainly  many  hundred  feet;  yet  this  differential  movement  has  been 
entirely  compensated  by  erosion,  and  there  has  been  stripped  from  the  northern 
half  a  crustal  layer  of  a  thickness  equal  to  the  sum  of  the  amount  of  the  displace- 
ment and  the  thickness  of  the  material  removed  from  the  southern  half  during 
the  same  period.  Similarly,  the  individual  faults,  elsewhere  considered,  show  that 
erosion  has  compensated  for  their  dislocations. 

FEATURES  OF  EROSION  Ilf  ARID  CL.IMATES. 

In  the  arid  Great  Basin  region  the  conditions  governing  the  origin  of  topo- 
graphic forms  are  different  from  and  more  complicated  than  those  which  exist  in 
well-watered  regions,  where  most  of  the  reliable  physiographic  conclusions  have 
been  formulated.  It  is  therefore  important  that  in  a  region  where  the  topography 
is  well  mapped  and  the  geology  fairly  well  understood,  as  in  Tonopah,  the  origin 
of  the  forms  should  be  examined. 

The  writer  has  previously  remarked  that  in  the  greater  part  of  the  arid  Great 
Basin  region  the  effect  of  the  scant  moisture  as  an  agency  of  erosion  is  equaled  or 
exceeded  by  disintegration,  gravity,  and  wind  action,  with  the  result  that  in  the 
lower  valleys  leveling  instead  of  dissection  is  brought  about,  and  in  the  higher  ones 
dissection  is  much  less  marked  than  in  moister  regions."  The  general  conclusions 
reached  by  the  writer  concerning  processes  of  erosion  in  the  Great  Basin  region, 
as  expressed  in  an  unpublished  paper  read  before  the  Geological  Society  of 
Washington  in  the  spring  of  1903,  are  as  follows: 

Climate  controls  not  only  the  speed  of  erosion,  but  its  manner.  In  moist 
climates  the  precipitated  moisture  gathers  into  permanent  bodies  of  running  water, 
a  stream  system  is  maintained,  and  erosion  goes  on  chiefly  along  these  lines.  Thus 
even  those  rocks  which  offer  no  differences  in  weakness  are  thoroughly  dissected,  not 
because  the  materials  in  the  valleys  are  less  resistant,  but  because  there  the  eroding 
activity  is  concentrated.  The  disintegrated  rock  or  soil,  except  along  these  naked 
stream  beds,  is  cemented  with  moisture  and  bound  together  by  vegetation,  and  so 

a  Bull.  Geol.  Soc.  Am.,  vol.  12.  p.  237. 


112  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 

is  fairly  well  armored  against  the  attacks  of  erosion,  which  can  make  but  compara- 
tively slow  progress. 

In  a  truly  arid  region,  where  there  are  extremes  of  heat  and  cold,  rock 
disintegration  at  the  surface  is  much  more  rapid.  Streams  are  rare,  transient, 
and  relatively  unimportant,  and  stream  erosion  is  slight  compared  with  that  of 
moister  regions.  Yet  erosion  is  active,  so  that  in  the  Great  Basin  region  even 
moderately  steep  slopes  are  stripped  of  debris  and  consist  of  hard,  unweathered 
rock.  The  lack  of  vegetation  renders  the  whole  surface  equally  susceptible  of 
attack  by  frosts,  thaws,  rains,  and  snows,  and  the  disintegrated  material  creeps  by 
the  nearest  way,  in  the  form  of  a  sheet,  into  the  depressions.  Thus  the  fronts  of 
many  of  the  Basin  ranges  are  bordered  by  a  continuous  apron  of  debris  sloping 
down  into  the  center  of  the  valley,  an  enormous  mass  of  waste  which  is  relatively 
slightly  increased  by  the  alluvial  fans  at  the  mouths  of  the  gulches  (PI.  XV,  £). 

In  desert  regions  the  more  nearly  equable  distribution  of  the  eroding  agents 
causes  the  differences  in  hardness  of  the  attacked  rocks  to  be  far  more  prominent  in 
determining  the  lines  of  relief.  In  proportion  as  the  aridity  increases  the  topo- 
graphic forms  show  more  and  more  faithfully  the  resistance  of  the  rocks.  If  the 
rocks  are  folded  and  faulted  the  ridges  will  follow  the  lines  of  strike  and  of  faulting. 
In  a  country  of  igneous  rocks  a  new  element  is  introduced,  but  here  also  erosion 
tends  to  preserve  the  original  lines  of  structure.  In  intervals  of  moister  climate 
streams  will  cut  gorges,  a  tendency  which  is  probably  antagonized  in  succeeding 
arid  periods. 

It  is  proper  to  insist  here  that  these  distinctions  apply  to  truly  arid  climates, 
and  are  more  applicable  as  the  aridity  increases.  Semiarid  regions,  where  violent 
rains  are  not  infrequent,  have  a  different  topography.  The  abundant  waters  of  the 
storms  flow  down  the  slopes  in  rushing  torrents,  which  cut  their  beds  all  the  more 
deeply  because  the  rock  is  naked  and  disintegrated  as  the  result  of  the  intervening 
periods  of  aridity.  A  rugged,  well-dissected  topography  may  sometimes  result, 
often  wrongly  described  as  typical  of  arid  regions. 

PRECIPITATION"  IX  REGION  NEAR  TONOPAII. 

Although  violent  rains  sometimes  occur  at  Tonopah,  especially  in  the  spring, 
they  are  rare,  and  the  region  can  not  be  classed  as  semiarid;  it  approaches  more 
nearly  true  aridity,  and  the  channeling  by  torrents  is  not  so  important  as  the 
universal  downward  working  of  disintegrated  material. 

No  records  of  precipitation  have  been  kept  in  Tonopah,  for  the  town  is  only 
a  few  years  old.  The  observations  made  by  the  United  Sates  Weather  Bureau 
in  Sodaville,  60  miles  farther  northwest,  are  as  follows: 


U.    S.    GEOLOGICAL  SURVEY 


PROFESSIONAL    PAPER    NO.   42       PL.    XV 


RECENT    BASALTIC    CONE    NEAR   SILVER    PEAK. 


B.     EAST   FRONT  OF   QUINN    CANYON    RANGE,   SHOWING   WASH   APRON   TYPICAL  OF   REGION. 


RELATION    OF    RELIEF    TO    ROCK    RESISTANCE.  113 

Precipitation,  in  inches,  at  Sodarille,  Nev. 

1898 4.72 

1899 2.30 

1902 1.68 

1903 2.16 

Average  for  these  four  years  (others  not  completely  observed),  2.71   inches. 

DEPENDENCE  AT    TONOPAH   OF    TOPOGRAPHIC    RELIEF   UPON    ROCK 

RESISTANCE. 

After  the  great  amount  of  erosion  which  the  Tonopah  district  has  undergone, 
the  relief  is  to-day  determined  in  a  very  remarkable  way  by  the  character  of 
the  rocks.  The  relief  here  is  not  like  that  resulting  from  the  work  of  stable 
and  .strong  streams,  concentrating  and  almost  monopolizing  the  erosion,  pushing 
back  their  systematic  valleys  from  one  rock  formation  into  another  and  constantly 
broadening  their  domains.  It  is  rather  like  that  produced  by  the  warm  breath 
of  the  sun  on  a  mass  of  ice  and  snow,  where  the  softer  material  fades  into  the 
air  and  the  harder  skeleton  of  ice  protrudes  above  the  surface. 

The  most  prominent  topographic  features  in  the  Tonopah  district  are  the 
denuded  volcanic  necks — such  as  Butler,  Brougher,  Golden,  Siebert,  and  Ararat 
mountains,  and  Mount  Oddie — where  the  hard  lava  column  has  resisted  erosion, 
while  the  surrounding  softer  material  has  been  worn  down.  The  map  shows  how 
closely  the  contours  conform  to  the  irregularities  of  the  intrusion  and  to  how 
great  a  degree  the  difference  of  resistance  has  controlled  even  minor  features  of  the 
topography.  Around  the  margins  of  the  white  (Oddie)  rhyolite  intrusions  very 
well-marked  and  closely  set  division  planes  parallel  to  the  contact  (platy  structure) 
render  these  border  zones  often  more  easily  attacked.  The  outlying  rhyolite 
dikes  also  show  this  markedly,  so  that  (as  around  Mount  Oddie)  such  dikes,  when 
relatively  narrow^ave  been  easily  degraded  to  the  level  of  the  intruded  rocks. 

The  smaller  eminences  are  also  almost  always  due  to  a  harder  intrusive  rock,  as, 
for  example,  Heller  Butte.  The  intrusive  glassy  Tonopah  rhyolite-dacite  in  the 
northern  portion  of  the  area  mapped  is  evidently  harder  than  the  intruded  later 
andesite  and  occupies  in  general  higher  ground.  Study  of  the  map  shows  how 
outlying  bodies  of  this  rhyolite-dacite  are  frequently  responsible  for  hills  and 
ridges,  while  depressions  have  formed  along  the  strips  of  later  andesite  flanked  on 
the  sides  by  the  rhyolite-dacite. 

Mizpah  Hill  and  Gold  Hill  an  -.  fault  blocks  whose  relative  relief  is  due  to  the 

greater  resistance  of  the  silicified  earlier  andesite.  of  which  they  are  made  up,  as 

compared  with  surrounding  rocks.     On  the  east  of  Mizpah  Hill,  where  the  adjacent 

rocks  are  the  soft  lake  beds,  the  scarp  is  fairly  well  developed;  on  the  west  side  the 

16843— No.  42—05 8 


114  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,   NEVADA. 

difference  of  resistance  between  the  earlier  and  the  later  andesite  is  not  great,  so 
that  the  slope  is  more  uniform.  The  fact  that  the  contours  on  the  southwest  corner 
of  the  Mizpah  Hill  fault  block  are  parallel  to  the  contact  of  the  softer  lake  beds 
shows  the  minuteness  with  which  the  relief  has  been  determined  by  the  rock 
resistance. 

EFFECTS  OF  FAULTING  UPON  THE  TOPOGBAPHT. 

The  effect  of  faulting  on  the  topography  in  general  is  comparatively  unimpor- 
tant. The  two  earlier  andesite  hills  above  mentioned  are  the  most  conspicuous 
cases  where  faulting  has  been  (though  indirectly)  a  factor.  The  volcanic  (dacitic 
and  rhyolitic)  agglomerates  and  tuffs,  which,  by  the  accidents  of  faulting,  usually 
adjoin  faulted  blocks  of  the  Siebert  tuffs  in  the  southern  half  of  the  map,  are 
not  much  harder  than  these.  Nevertheless,  the  lake  beds  (Siebert  tuffs)  are 
undoubtedly  the  most  easily  eroded  of  all  the  formations,  and  areas  occupied  by 
them  are  characterized  for  the  most  part  by  a  smooth,  flat  surface,  bounded 
frequently  by  a  slight  scarp  where  the  tuff  adjoins  more  resistant  rock.  In  most 
cases,  however,  as  is  shown  by  the  map,  the  tuff  block  is  surrounded  on  all  sides  by 
the  harder  blocks,  and  as  there  is  no  outlet  for  eroded  material  the  tuff  block  can 
not  be  now  much  below  the  harder  blocks. 


CHAPTER  V. 

DESCRIPTIVE  GEOLOGY  OF  MINES  AND  PROSPECTS. 
THE  KNOWIST  EARLIER  ANDESITE  VEINS. 

MIZPAH  VEIN   SYSTEM. 
MIZPAH    VEIN. 

EXTENT   OF    VEIN. 

Limitation  of  vein  Jyy  Mizpah  fault. — The  Mizpah  vein  has  a  strong  outcrop 
(PI.  XVII),  extending  for  a  distance  of  about  800  feet  in  a  nearly  due  east-west 
direction.  Toward  the  east  end  it  is  broken  by  a  number  of  small  faults,  mostly, 
it  appears,  with  a  north-south  strike  and  an  easterly  dip,  by  which  the  vein  is 
offset,  now  in  one,  now  in  the  other  direction;  and  it  is  cut  off  abruptly  by  the 
great  northwest-southeast  break,  which  may  be  called  the  Mizpah  fault  (PI. 
XVI).  This  fault  is  clearly  recognizable  at  the  surface  and  in  the  underground 
workings  on  the  several  levels  (PI.  XIX),  as  well  as  in  the  Desert  Queen,  the 
Montana  Tonopah,  and  the  North  Star  workings.  It  has  a  moderate  dip  to  the 
northeast.  Wherever  the  veins  have  been  followed  to  this  fault,  they  have  been 
found  to  be  cut  off  abruptly;  and  the  presence  of  a  zone  of  clay  due  to  rubbing 
or  trituration,  and  frequently  of  a  drag  of  fragments  from  the  quartz  veins 
along  this  zone,  give  evidence  of  a  great  movement  subsequent  to  the  vein 
formation.  On  the  upper  side  of  the  -fault,  overlying  the  earlier  andesite  in 
which  the  veins  lie,  is  the  later  andesite.  Since  the  later  andesite  is  normally 
above  the  earlier  andesite,  a  normal  fault,  with  a  downthrow  on  the  northeast 
side,  is  shown. 

Limitation  of  vein  by  Burro  fault. — On  the  west  side  the  outcrop  of  the  vein  is 
abruptly  cut  off  by  the  Burro  fault,  beyond  which  the  later  andesite  again  outcrops. 
This  fault,  traceable  on  the  surface  only  by  the  use  of  the  utmost  skill,  runs 
northeastward.  A  break  corresponding  to  this,  and  probably  identical  with  it, 
has  been  encountered  underground  in  the  west  workings  of  the  mine,  farther 
west  than  on  the  surface,  showing  that  the  fault  dips  northwestward. 

Limitation  of  vein  by  Siebert  fault. — The  vein  normally  dips  north  at  an  average 
steep  angle,  but  alternately  flattens  and  steepens,  and  is  even  locally  overturned 
(fig.  16).  It  has  been  followed  on  the  dip  to  a  depth  of  nearly  700  feet,  where  it 

115 


116 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 


is  cut  off  by  nearly  flat  fault,  which  dips  west  at  a  moderate  angle.  This  may  he 
called  the  Siebert  fault  (fig.  17).  On  the  upper  side  of  the  fault  the  rock  is  chiefly 
light-colored,  partly  oxidized,  silicified  earlier  andesite,  mixed  with  much  barren 
quartz;  on  the  lower  side  it  is  unoxidized  and  has  a  different  appearance,  and 
though  study  shows  it  to  be  probably  the  earlier  andesite  there  is  much  chlorite 
and  sometimes  calcite  among  its  decomposition  products,  thereby  separating  it 
sharply  from  the  andesite  inclosing  the  veins,  which  has  characteristically  been 


Surface 


Surface 


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Scale 
o        10       20      30      40       so  feet 

H;.  lf>. — Vt-rticiil  cross  section  of  a  portion  of 
Mi/pith  vein  as  exposed  in  the  Oddie  shaft, 
showing  reversed  dip  near  the  surface. 


Wm^&8&&i^&&?&  m*.,-'"v 


Scale 
o       so       mo  200  300  feet 

FIG.  17. — Vertical  cross  section  of  Mizpah  vein  along 
Brougher  shaft  and  inclines. 


altered  to  quartz  and  sericite.  Below  the  700-foot  level  in  the  Siebert  shaft 
(PI.  XVIII)  this  rock  in  places  is  altered  chiefly  to  quartz  and  sericite,  and  ev<-ii 
contains  silicified  zones  or  quartz  veins  giving  assays  of  a  few  dollars  to  the  ton. 
At  a  depth  of  about  935  feet  there  was  encountered  a  body  of  dacitic  or  rhyolitic 
rock  resembling  the  rock  in  the  lower  part  of  the  Mizpah  Extension,  and  probably 
referable  to  the  Tonopah  rhyolite-dacite;  below  this  a  vertical  drill  hole  shows  that 
the  same  rock  is  continuous  to  nearly  1,400  feet  from  the  surface,  where  the  boring 
was  stopped. 


U.   S.   GEOLOGICAL  SURVEY 


OUTCROPPING   VEINS   OF   MIZPAH    HILL 


MIZPAH    VEIN. 


117 


On  the  700-foot  level,  south  drift,  above  the  Siebert  fault,  there  was  encoun- 
tered a  higher  body  of  Tonopah  rhyolite-dacite  containing  some  barren  quartz, 
and  similar  rock  occurs  on  an  east  drift  on  the  same  level.  An  east  drift  on  the 
500-foot  level  ran  into  a  mass  of  the  same  formation. 


VEIX    STRUCTURE. 


The  Mizpah  vein  is  usually  several  feet  wide.  Its  walls  are  always  earlier 
andesite,  which  is  generally  completely  altered  to  quartz,  sericite,  etc.  The  vein 
may  be  succinctly  described  as  a  sili-  ^  S 

cified  and  mineralized  sheeted  zone  in 
the  andesite.  There  are  all  stages  of 
transition  from  the  sheeted  altered  an- 
desite (PI.  XX)  to  solid  quartz.  Both 
extremes  may  be  observed  at  many 
places  along  the  vein,  and  sometimes 
not  very  far  apart.  More  frequently 
the  vein  is  intermediate  in  character, 
showing  a  variable  amount  of  quartz  in 
the  altered  porphyry.  Sometimes  the 
quartz  forms  parallel  streaks  or  vein- 
lets  and  sometimes  it  occurs  reticulated 
in  the  decomposed  rock.  Frequently 
some  of  these  small  veinlets  possess  comb 
structure,  which  shows  that  the3r  origi- 
nated by  deposition  in  open  cavities;  but 
their  frequently  irregular  branching  and 
their  distribution  indicate  that  these  cavi- 
ties were  caused  by  solution  by  circulat- 
ing waters  and  not  by  fracturing.  Their 
very  existence  proves  that  the  main 
zone  did  not  originate  by  fracturing. 
As  a  rule,  however,  even  the  small 
veinlets  give  no  evidence  of  having  been  deposited  in  cavities,  but  have  evidently 
been  formed  by  a  process  of  silicitication  of  the  porphyry  involving  replacement, 
the  extreme  of  the  process  which  has  altered  the  andesite  near  the  veins.  This 
profound  alteration  of  the  zone  which  became  the  vein  was  caused  by  close-set 
parallel  fractures,  which  marked  this  zone,  and  which  afforded  a  favorable  channel 
for  the  silicifying  and  mineralizing  solutions. 

The  main  premineral  fractures  had  therefore  the  course  of  the  present  vein, 
striking  east  and  west  and  dipping  steeply  north.  Frequently,  also,  the  walls  are 
locally  not  parallel  (fig.  18). 


Scale 
10 


20  feet 


FIG.  18.— Vertical  cross  section  of  Mizpah  vein.  Oddieand 
McMann  lease,  showing  diverging  walls. 


118 


GEOLOGY   OF  TONOPAH    MINING   DISTRICT,  NEVADA. 


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Scale  , 

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Flo.  19.— Detail  sections  from  Mizpah  vein  showing  the  effect  of  premineral  cross  fractures.  A,  Sketched  vertical  cross  sect  ii  m 
of  Mizpah  vein  at  a  point  In  the  west  workings  (the  whole  vein  sloped  out).  B,  Sketched  vertical  cross  section  of 
Mizpah  vein,  Oddie  and  McMann  lease.  C,  Detail  of  Mizpah  vein  in  the  Golden  big  stope,  about  160  feet  from  the 
surface.  Hori/xmtal  plan  along  a  crosscuttlng  fracture  vein;  the  vein  abruptly  increases  in  width  from  2J  to  10  feet. 
Fracture  dips  43°  southwest,  and  the  vein  is  here  perpendicular.  There  is  no  evidence  of  movement  along  the  cross- 
cutting  fracture,  but  it  appears  to  be  premineral.  Its  function  as  a  fracture  plane  limiting  the  circulation  is  like  that 
of  a  wall.  It  may  therefore  be  called  a  cross  wall.  />,  Cross  section  of  Mizpah  vein,  west  face  of  big  stope,  Lynch 
and  Omcara  workings,  about  KX)  feet  from  the  surface,  illustrating  the  influence  of  crosscutting  fractures  on  the 
original  ore.  The  ore,  which  was  a  solid  mass  east  of  here,  is  cut  off  ulong  u  premineral  fracture  plane  (strike  N.  56°  E., 
dip  60°  northwest),  above  which  the  vein  is  divided  Into  a  foot-wall  and  a  hanging-wall  streak,  with  altered  andesitc 
between. 


MIZPAH  VEIN.  119 


EFFECTS   OF  TRANSVERSE    PREMIXERAL    FRACTURES. 


There  were  also  minor  fractures,  among  which  some  striking  in  a  general  north- 
south  direction,  and  dipping  east,  can  be  recognized. 

Cross  walls. — These  are  shown  by  jogs  in  the  vein  following  these  planes,  or,  very 
frequently,  by  a  change  from  a  highly  silicified  or  mineralized  condition  to  a  less 
altered  one,  while  the  vein  zone  is  continuous  and  undisturbed.  These  jogs  or  offsets 
may  occur  on  both  sides  of  the  vein,  and  thus  may  simulate  faults,  from  which  they 
are  distinguished  by  the  lack  of  evidence  of  movement;  or  they  may  be  restricted  to 
one  side  of  the  vein  (fig.  19),  in  which  case  they  can  not  be  mistaken.  Sometimes  the 
vein  may  jog  in  opposite  directions  on  the  two  sides  of  such  a  critical  cross  plane  or 
premineral  fracture,  and  so  become  markedly  larger  or  smaller  (fig.  19,  B). 

Branching  veins. — Small  veins  which  diverge  from  the  main  vein  also  testify  to 
these  crosscutting  premineral  fractures. 

Besides  the  north-south  premineral  fractures,  there  were  other  fractures 
having  a  variety  of  strikes  intermediate  between  that  of  the  main  vein  zone  and 
the  cross  fractures.  Among  other  things  this  is  evidenced  by  the  portions  of 
the  veins  which  split  up  from  the  main  vein  and  reunite  with  it.  This  splitting 
and  reuniting  takes  place  in  both  a  horizontal  and  a  vertical  direction,  and  the 
general  result  can  best  be  explained  by  illustrations  (fig.  20).  The  veins  thus  belong 
to  the  class  of  linked  veins,  and  this  same  relation  is  exhibited  on  a  larger  scale 
between  some  of  the  larger  veins,  and  will  be  described. 

The  intersections  of  the  minor  veins  with  the  vein  zone  seem,  as  a  rule,  to 
pitch  to  the  east  also,  like  the  main  crosscutting  premineral  fractures. 

Origin  of  ore  shoots. — The  main  crosscutting  fractures,  striking  north  and 
south  and  dipping  east,  as  above  explained,  in  many  places  separate  the  highly 
silicified  and  mineralized  vein  zone,  often  by  a  sharp  division,  from  a  portion 
which  has  not  been  so  much  altered.  These  richer  portions  ma}-  be  considered 
ore  shoots;  and  while  their  internal  size  and  richness  are  very  irregular,  a 
careful  plotting  of  the  results  of  the  assay  chart"  shows  that  the  richer  portions 
of  the  vein  may  be  separated  into  broad  east-dipping  shoots,  of  which  there  are 
three  within  the  developed  vein.  The  internal  distribution  of  the  ore  in  these 
shoots  would  make  an  interesting  study  if  enough  data  were  on  hand. 

Fig.  21  shows  the  shoot-like  distribution  of  the  richest  portions.  The  space 
between  and  beyond  the  shoots  is,  however,  good  ore.  The  company  does  not 
desire  to  have  the  figures  published,  but  it  may  be  said  that  the  amount  of  gold 
and  silver  in  those  parts  of  the  vein  left  blank  on  the  diagram  is  fully  equal  to 
that  contained  in  the  greater  part  of  the  ore  produced  by  the  Comstock  during 

a  Kindly  furnished  to  the  writer  by  the  Tonopah  Mining  Company. 


120 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 


Lynch  frO'Meara  shaft     xx 


Scale  ,. 

50  75 100  teet 


FIG.  'A).— Sections  to  show  the  splitting  of  the  Mizpah  vein.  -4,  Horizontal  plan  of  portion  of  Mizpah  vein  as  developed 
on  the  260-foot  level,  Mizpah  mine,  east  of  the  Brougher  shaft.  11,  Horizontal  plan  of  portion  of  Mizpah  vein  as 
exposed  in  the  Golden  and  Kendall  and  McMann  leases,  about  130  feet  below  the  surface,  showing  splitting  of  solid 
vein  into  foot-wall  and  hanging-wall  seams.  C,  Horizontal  sketch  section  of  Mizpah  vein,  big  slope.  Lynch  and 
Omeara  workings,  showing  splitting  of  vein  in  two.  1),  Vertical  sketch  cross  section  of  a  portion  of  the  Mizpah 
vein  at  the  Clark  shaft,  from  the  surface  down.  Junction  of  veins  (a)  pitches  east  on  the  vein  at  an  angle  of  about  45°. 
E,  Horizontal  section  of  same,  taken  at  60  feet  below  the  surface. 


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M1ZPAH    VEIN. 


121 


its  best   days."    This    whole  plan,  therefore,  shows,  in    the   large  sense,  a  single 
bonanza,  comparable  in  size  and  richness  to  those  of  the  Comstock  (tig.  78,  p.  277). 
As  a  rule,  within  the  mine,  great  size   of   the  vein  coincides  with  increased 
richness,  although  exceptionally  this  is  not  true. 


W 


Surface 


100 


B 

Scale 
zoo 


oo  feet 


FIG.  21.— Diagram  showing  the  distribution  of  the  richer  ores  in  the  Mizpahvein.  A,  Distribution  of  the  richer  ores  as  indi- 
cated by  assays;  black  dots  indicate  assays  above  a  certain  figure;  lower  assays  not  indicated.  Diagram  of  the  Mizpah 
vein  projected  on  a  vertical  plane.  The  diagram  indicates,  roughly  outlined,  broad  eastward-pitching  shoots  of  rich 
ore.  B,  Mizpah  vein  projected  on  a  vertical  section,  showing  slopes  from  which  ore  has  been  removed  above  300-foot 
level.  Previous  to  the  making  of  the  assay  plan  (fig.  21,  A)  the  distribution  of  these  slopes  indicated  the  eastward- 
pitching  richer  shoots. 

From  what  has  been  said,  it  is  seen  that  the  ore  shoots  are  primary.  Along 
certain  portions  of  the  east-west  fracture  zone  (those  portions  being  governed  by 
north-south  striking  and  east-dipping  fractures)  the  circulation  of  mineralizing 


aMon.  U.  S.  Geol.  Survey,  vol.3,  p.  10. 


122 


GEOLOGY    OF   TONOPAH   MINING    DISTRICT,   NEVADA. 


waters  has  been  freer  and  the  result  greater.  In  these  portions  the  channels  must 
have  been  more  open,  and  since  the  main  vein  zone  was  a  single  set  of  fractures, 
with  fairly  uniform  conditions,  the  difference  in  degree  of  openness,  influencing 
circulation,  must  have  depended  largely  on  the  cross  fractures.  In  other  words,  it 
appears  likely  that  whore  these  cross  fractures  were  most  numerous  rude  east-dipping 
columns  or  chimneys,  speaking  general!}',  were  fonhed,  in  which  the  circulat- 
ing solutions  were  relatively  concentrated. 

POSTMINEEAL   FAULTS   AND   FRACTURES. 

Postmineral  fractures  and  faults  are  com- 
mon. Besides  the  great  faults  mentioned 
there  are  continually  encountered  in  the  mine 
many  minor  ones  (figs.  22  and  23)  which  may 
prove  puzzling  to  the  miner.  Small  faults 
are  very  numerous  in  the  workings  in  the 
vicinity  of  the  great  Mizpah  fault.  These 
faults  usually  strike  north-south  and  dip 
east,  though  they  may  have  other  attitudes. 
Numerous  postmineral  fractures,  along  which 
there  has  been  no  movement,  have  the  same 
general  north-south  strike  and  easterly  dip, 
while  others  have  a  variety  of  positions  (fig. 
24).  Postmineral  fractures  parallel  to  the 
vein  are  always  present.  In  other .  words, 
the  postmineral  fractures  in  general  have  the 
same  directions  as  the  premineral  fractures,  and  stress  subsequent  to  the  ore 
deposition  has  reopened  the  old  wounds,  which  had  been  more  or  less  completely 
healed  by  the  vein  formation  (fig.  25). 

VEIN   COMPOSITION. 

The  quartz  of  the  vein  is  fine  and  cloudy.  Poor  quartz  and  rich  quartz  are 
often  much  alike  in  appearance,  save  for  a  purplish  tinge  in  the  latter.  Under 
the  microscope  this  tinge  is  seen  to  be  due  to  disseminated  particles  of  argentite. 
This  mineral  is  found  from  the  outcrop  of  the  vein  downward,  through  all  the 
oxidized  zone.  Silver  chloride  is  also  very  abundant,  though  usually,  like  the 
other  metallic  minerals,  it  is  determinable  only  microscopically.  Orange  and 
yellow  amorphous  minerals  were  also  observed,  and  surmised  to  be  the  combinations 
of  silver  with  chlorine,  bromine,  and  iodine,  and  chemical  examination  of  the  speci- 
mens showed  the  presence  of  all  these  elements.  Free  gold  is  sometimes  observed, 


Scale 


z  feet 


FIG.  22. — Sketch  of  faulted  quartz  veinlets  in  ancles. 
ite,  300-foot  level,  Mizpah,  just  south  of  the  Valley 
View  shaft. 


u.  s.  GEOLOGICAL  SURVEY 


PROFESSIONAL    PAPER    NO.   42      Pl~   XX 


mmz&ti 
iPSPSI 

'£<ig5z!%££*s* 


— -  ••{         > 

'  <•  tS>  vV.  \  V I  >  ,-.v  7;V.  I  •  J 


Scale  for  sections  (A)-(B) 


CROSS  SECTIONS,  SHOWING  STRUCTURE  OF   MIZPAH   VEINS. 

A.     Sketch  of  Mizpah  vein,  vertical  cross  section,  300-foot  level,  west  drift. 

R.     Cross  section  of  portion  of  Mizpah  vein  in  Tuscarora  stopes,  about  90  feet  from  the  surface. 
C.     Vertical  cross  section  of  portion  of  Mizpali  vein,  300-foot  level,  west  drift.  200  feet  west  of  shaft. 
/>.     Mizpah  vein,  300-foot  level,  east  drift,  near  main  crosscut. 
E.     Sketch  of  vein  in  crosscut  south  from  east  drift,  300  foot  level,  Mizpah  mine. 

I  rock,  decomposed  andesite;  6  =  highly  fractured,  decomposed,  and  silicified  andesite;   c~ granular,  gray,  aluminous  quartz  (ordinary  variety  of  these 
veins);  d  —  clear  quartz,  filling  cavities,  often  with  corrb  structure. 


MIZPAH    VEIN. 


123 


especially  under  the  microscope.  A  slight  copper  stain  has  been  reported  on  the 
ore,  but  the  writer  has  never  seen  any.  Ruby  silver  and  argentite  sometimes  occur 
on  cracks,  but  as  a  rule  these  minerals,  if  present,  are  not  visible  to  the  naked  eye. 


Scale 
o 5 io         15         20        ?5  feet 

Fio.  23.— Horizontal  sketch  plan  of  portion  of  the  Jlizpah  vein  in  slopes  east  of  Lease  52,  about  70  feet  from  surface, 

showing  probable  compensating  faulting. 


FIG.  24. — Reproduction  of  drawing  of  model,  showing  the  principal  postmineral  fractures  and  faults  observed  in  the 
Mizpah  mine  workings.  The  strikes  of  these  fractures  have  been  plotted  through  a  center  point  on  the  top  of  the 
cube,  and  the  intersection  of  the  fractures  with  the  other  faces  of  the  cube  has  been  drawn.  The  endless  variety  of 
patterns  which  are  made  by  the  same  systems  of  fracturing  by  their  intersection  with  different  planes  is  here  shown. 
A,  Front  view  of  block,  looking  down.  B,  Rear  view  of  block,  looking  up. 

Black  manganese  oxide  is  frequent  and  often  concentrated  in  little  vugs.  Iron  oxide, 
the  result  of  the  alteration  of  pyrite,  occurs,  and  sometimes  pyrite  itself,  but  this 
mineral  is  much  less  abundant  in  the  veins  than  in  the  wall  rock. 


124 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 


SECONDARY  NATURE  OF  ORE  MINERALS. 


It  is  probable  that  all  these  metallic  minerals  are  nearly  always  secondary. 
Ruby  silver  and  argentite  are  often  observed  in  this  camp  as  secondary  minerals 
coating  cracks,  as  well  as  horn  silver  (silver  chloride)  and  the  bromides  and  iodides. 
The  free  gold  appears  probably  secondary.  In  a  few  microscopic  sections  studied 
argentite  has  formed  as  an  alteration  of  silver  chloride,  itself  probably  secondary. 


REARRANGEMENT    OF    VALUES    DURING    OXIDATION. 


00  feet 


All   the  ores  in   the   mine    are   oxidized   or   semioxidized,    for    the   zone   of 

oxidation  goes  down  to  the 
600-foot  level  or  below, 
beneath  which  the  vein  is 
cut  off  by  the  flat  Siebert 
fault. 

The  facts  that  the  ores 
in  their  present  form  are 

FIG.  25.— Horizontal  diagrammatic'  plan  of  Mizpah  vein  as  exposed  in  the  Oddie  largely    altered     and    that 
and  McMann  lease,  20  to  30  feet  below  the  surface.    Of  the  crosscutting  frac-  '     J 

tures  (dotted  lines)  limiting  the  ore,  as  is  shown,  some  are  evidently  premi-  many       postmineral      frac- 
neral  fractures  or  cross  walls,  and  some  postmineral  fractures.    In  the  latter 

case  it  appears  probable  that  in  some  cases  the  postmineral  fracture  has  origi-  tUl'CS  are    present   Suggest 
nated  by  the  continuation  of  movement  along  a  premineral  fracture. 

the  inquiry  as  to  how  tar 

the  values  have  been  rearranged  and  concentrated  during  the  alteration  process. 
A  study  of  the  assay  plan  of  the  mine  failed  to  show  any  decisive  change 
at  different  depths  in  the  relative  proportion  of  gold  and  silver,  the  chief 
metallic  minerals  present  in  the  ores.  A  more  accurate  statement  of  this 
investigation  may  be  of  interest. 

Proportion  of  gold  to  silver  in  Mizpah  vein. 


Lifts.a 

Number  of 
assays. 

Percentage  of 
gold  by  weight. 

Proportion  of 
gold  to  silver. 

Fi*st  50  feet         

22 

1.00 

1  to  100. 

Second  50  feet                               

42 

.86 

1  to  116. 

Third  50  feet                             

40 

.85 

Fourth  50  feet         

57 

.88 

Fifth  50  feet             

55 

.88 

- 

Sixth  50  feet                                     

72 

.95 

Seventh  50  feet                      

19 

.77 

Eighth  50  feet                

8 

.73 

1  to  137. 

"The  word  "lift"  is  here  used  to  designate  one  of  the  horizontal  zones  into  which  the  vein  and  mine  have  been 
divided  for  the  purpose  of  measurement.  The  use  of  the  word  is  similar  to  that  in  speaking  of  the  different  "lifts"  of 
leather  on  a  shoe  heel,  and  the  writer  is  under  the  Impression  that  the  word  in  in  use  by  mining  engineers  with  the  same 
significance  as  given  above. 

In  value  the  gold  constitutes  25  to  30  per  cent  of  the  ore. 


DESERT    QUEEN    SHAFT.  125 

The  percentage  of  gold  may  be  smaller  in  the  lower  lifts,  but  the  data  are 
not  sufficient  to  support  this  idea,  and  a  proportion  similar  to  the  average 
(1:100)  has  been  found  in  the  shipments  of  primary  sulphides  from  the  rich 
ores  of  the  Montana  Tonopah. 

Moreover,  the  rich  shoots  seem,  under  microscopic  stud}7,  to  be  original, 
though  the  ore  is  largely  altered — that  is,  the  ore  seems  to  have  altered  essentially 
in  place,  without  any  thorough  rearrangement.  This  may  be  ascribed  in  part  to 
the  relatively  scanty  supply  of  surface  waters  in  this  arid  region. 

Some  transportation,  nevertheless,  was  inevitable,  and  it  is  probable  that  to 
a  minor  degree  the  ores  have  been  redeposited.  The  result  has  probably  been 
that  values  are  more  evenly  distributed  over  the  oxidized  vein  than  they  were 
originally;  and  the  vein  has  been  enriched  to  some  degree  by  the  downward 
penetration  of  minerals  leached  from  the  outcrop  as  it  was  eroded. 

GEOLOGY   OF   THE    DESERT  QUEEN    SHAFT. 

The  Desert  Queen  is  the  chief  working  shaft  of  the  Belmont  Company,  and 
the  ores  discovered  in  the  workings  from  this  shaft  are  usually  referred  to  as 
the  Belmont  ore  bodies.  The  shaft  is  one  of  the  deepest  in  the  camp. 


INTRUSIVE    NATURE    OF   RHYOLITE   CONTACT. 


As  shown  on  the  map  (PI.  XVI),  the  Desert  Queen  shaft  starts  in  the  rhyolite, 
on  the  southeast  slope  of  Mount  Oddie.  It  passes  downward  through  this  rhyolite 
for  250  feet,  below  which  it  encounters  a  mass  of  dark-blue  or  brown  clay, 
containing  harder  residual  bowlders  of  the  later  andesite.  This  has  a  thickness 
of  more  than  50  feet  in  the  shaft,  and  is  evidently  a  broken  and  ground  up 
later  andesite,  altered  to  a  clay  by  traversing  waters.  Below  this  water  occurs 
along  a  fracture. 

This  zone  of  movement  strongly  resembles  a  fault  zone.  However  it  is  to 
be  noted  that  the  movement  has  affected  only  the  andesite  and  not  the  rhyolite, 
that  the  rhyolite  is  not  noticeably  decomposed,  and  that  there  are  no  rhyolite 
fragments  in  the  breccia.  This  indicates  rather  that  the  disturbance  was  caused 
bv  the  intrusion  of  the  rhyolite  into  the  andesite.  The  exact  attitude  of  the 
rhyolite  contact  could  not  be  observed,  but  it  may  be  assumed  to  be  roughly 
parallel  to  the  watercourse  just  mentioned,  which  strikes  north  and  south  and 
dips  east  at  an  angle  of  60 D. 

At  the  surface  this  rhyolite  is  in  contact  with  the  Siebert  tuff  lake  beds 
about  120  feet  west  of  the  shaft,  as  shown  on  the  map;  this  contact  strikes  north 
and  south.  Beneath  the  lake  beds  in  this  block  lies  the  later  andesite,  as  shown 
for  example  in  the  Silver  Top  shaft,  a  short  distance  to  the  southwest.  A  line 


126 


GEOLOGY   OF   TONOPAH   MINING    DISTRICT,  NEVADA. 


drawn  from  this  rhyolite-andesite  contact  at  the  surface  near  the   Desert  Queen 
to  the  contact  in  the  shaft  has  a  general  angle  of  dip  of  about  68°.     The  contact 

at  the  surface  is  evidently  an  intrusive 
one,  being  on  the  western  side  of  one 
of  the  intrusive  lobes  which  radiate 
from  the  main  rhyolite  mass  of  Mount 
Oddie. 


Tonopah  Mining  Co. 
SteOcrt  shaft 


\  M.zpah  500  ft  leve 


Desert  Queen 


I     »'3r 

I   Belmont  609  ' 
Mf     uoor,,..*.. 


Scale 
?oo 


ofeet 


VARIABLE   ATTITUDE    OF   MOUNT    ODDIE    INTRUSIVE 
CONTACT. 

The  steep  dip  of  the  contact  be- 
tween the  andesite  and  the  intrusive 
rhyolite  at  this  point  is  in  contrast 
with  the  flat  portion  of  the  same  con- 
tact in -the  North  Star  shaft,  where  the 
lower  surface  of  the  rhyolite  is  very 
flat,  dipping  toward  the  mountain  at 
an  angle  not  greater  than  10°,  although 
the  later  andesite  shows  the  same  brec- 
ciation  as  at  the  contact  in  the  Desert 
Queen  shaft,  indicating  that  the  rhyo- 
lite is  intrusive.  This  difference  in  dip, 
however,  is  in  accord  with  other  obser- 
vations made  along  the  contact  of  the 
rhyolite,  all  possible  variation  being 
found,  the  contact  being  sometimes  flat, 
sometimes  vertical,  sometimes,  indeed, 
dipping  away  from  the  mountain  rather 
than  toward  it,  but  always  showing  the 
intrusive  character  of  the  rock. 

MIZPAH    VEIN    IN    DESERT  QUEEN    WORKINGS. 

The  Desert  Queen  shaft  cut  the 
Mizpah  fault  at  512  feet,  and  beneath 
it  the  earlier  andesite.  At  500  feet  a 

Fio.  26.— Horizontal  planof  mine  workings,  showing  the  rela-      drift   run   a   short   distance    north    from 
tlon  of  the  vein  In  the  Desert  Queen  workings  to  that  on  the      ±1          i      »i  .1         \i-    „    u     i-       1, 

corresponding  level  of  the  Mizpah  mine.  the   *haffc    cut    the    Mizpah    fault    again 

and  exposed  an  important  vein  (fig.  26). 

The  vein  is  sharply  cut  off  by  the  fault  on  the  east  and  is  much  sheeted  and  broken 
by  the  fault  movement,  so  that  its  course  is  not  immediately  evident.     The  general 


BURRO    VEINS.  127 

trend,  however,  appears  to  be  east  and  west,  or  perhaps  more  correctly  N.  70°  E., 
and  it  appears  to  have  a  steep  northerly  dip  and  a  width  of  several  feet.  It  is 
somewhat  dragged  in  the  neighborhood  of  the  fault.  Some  very  rich  ore  was 
found  here,  which  was  chiefly  oxidized  and  contained  a  large  amount  of  silver 
chloride.  The  Mizpah  fault  here  has  its  usual  northwest  strike  and  northeast  dip, 
but  the  dip  is  steeper  than  it  is  farther  northwest,  being  from  35°  to  45°. 

FORMATIONS   ENCOUNTERED   IN  THE  LOWER  WORKINGS. 

At  a  depth  of  814  feet  from  the  top  of  the  shaft  the  rocks  to  the  north  and 
west  are  extensively  explored  by  drifting.  These  workings  are  almost  entirely 
in  a  white,  dense  rock  which  study  shows  to  be  Tonopah  rhyolite-dacite.  The 
quartz  masses  characteristic  of  this  formation  were  encountered,  showing  the 
usual  irregularities,  nonpersistence,  and  barrenness;  on  the  other  hand,  some 
portions  are  exceptional  in  containing  high-grade  ore. 

At  a  depth  of  920  feet  the  shaft  cut  a  sheet  of  white  Oddie  rhyolite,  which 
contained  a.  flat  vein  of  some  size,  but  showed  no  values  of  importance  (see 
p.  193).  The  bottom  of  the  shaft,  at  a  depth  of  1,114  feet,  is  in  the  Tonopah 
rhyolite-dacite.  This  rock  is  much  like  that  at  the  bottom  of  the  Siebert  shaft. 

THE   BURRO  VEINS. 

On  the  south  side  of  the  Mizpah  vein  there  are  several  weaker  auxiliary  veins. 
Most  of  these  converge  and  unite  with  the  Mizpah  on  the  surface  at  a  point  not  far 
west  of  the  outcrop  of  the  Mizpah  fault.  The  principal  ones  have  been  called  the 
Burro  veins,  and  they  have  been  numbered  1,  2,  and  3,  No.  1  being  nearest  the 
Mizpah. 

These  three  veins  are  all  branches  of  the  system  of  which  the  Mizpah  is  the 
trunk  vein;  on  the  surface  the  No.  2  and  the  No.  3  probably  come  together  about 
200  feet  south  of  the  Mizpah;  this  united  vein  joins  the  No.  1  just  south  of  the 
Mizpah,  and  unites  with  the  trunk  a  very  short  distance  farther  northeast. 


VEIN    STRUCTURE. 


These  veins  are  all  essentially  silicifications  of  definite  fracture  zones  in  the 
altered  andesite.  The  zones  average  perhaps  4  feet  in  thickness,  and  along 
them  quartz  has  formed  (almost  entirely  by  replacement  of  the  altered  and 
silicified  andesite)  to  a  varying  degree,  so  that  in  places  the  vein  zone  may  be 
entirely  of  andesite,  distinguishable  from  the  wall  rock  only  by  its  peculiar 
and  greater  fracturing,  and  in  other  places  may  be  entirely  filled  with  quartz, 
carrying  good  values  in  silver  and  gold.  All  intermediate  stages  are  also  seen 
(fig.  27). 


128 


GEOLOGY    OF    TONOPAH    MIKING    DISTRICT,   NEVADA. 


Vein  rone 


-H          Surface 


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Wimj, 
iiiim 

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Wti--M>W 

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d        a        &         a      4  a 

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—  *• 

Surface 

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N'  *'-\  /    '  \  ':, 

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v1/ 

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.'.'    ^     *       X      -A"  - 

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1  '  /      '    v  /      ^  / 

\  .'V      ^  \-  .'  '  • 

I 

<-  ' 

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'  s      v    '  '**    '        i  - 

,  :'.  ::\    -•"•-.v':'s 

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'•  / 

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a    6 


Scale 


a  10  feet 


FIG.  27.— Sections  showing  thestructurc  of  the  Burro  Xo.  1  vein:  A,  Vertical  detailed  sketched  cross  section  of  a  portion  of 
Burro  No.  1  vein  at  the  surface,  as  exposed  by  a  prospecting  pit,  at  a  point  about  600  feet  west  of  the  probable  junction 
with  the  Mizpah  vein.  B,  Vertical  detailed  sketched  cross  section  of  a  portion  of  Burro  Xo.  1  vein  at  the  surface,  as 
exposed  by  surface  workings,  at  a  point  about  128  feet  west  of  section  A.  C,  Vertical  detailed  sketched  cross  sec- 
tion of  Burro  No.  1  vein,  as  exposed  at  the  surface,  at  a  point  6  feet  east  of  section  B,  showing  rapid  thinning  and 
disappearance  of  quartz  from  the  vein  zone.  D,  Detailed  sketched  vertical  section  of  Burro  No.  1  vein  at  a  point  about 
180  feet  west  of  sections  B  and  C,  showing  hanging-wall  and  foot-wall  veins  In  the  vein  zone.  E,  Vertical  detailed 
sketched  cross  section  of  a  portion  of  Burro  No.  1  vein,  as  exposed  at  the  surface  by  a  prospecting  pit,  at  a  point  about 
150  feet  west  of  section  I),  and  near  the  farthest  point  west  that  the  vein  has  been  traced,  showing  vein  zone  with 
only  hanging-wall  streak,  and  also  the  manner  of  dying  out  and  disappearance  of  this  class  of  veins.  F.  Horizontal 
sketched  plan  of  Burro  No.  1  vein,  uniting  the  two  vertical  sections  B  and  C,  showing  the  manner  in  which  the 
change  takes  place.  In  all  these  figures  a  filtered  andcsite;  b^q 


VALLEY    VIEW    VEIN    SYSTEM.  129 


STRENGTH    AND    EXTENT    OF   THE    BURRO    VEINS. 


Of  the  three  Burro  veins,  that  next  the  Mizpah,  No.  1,  is  the  strongest.  No.  2 
is  next,  and  No.  3,  the  farthest  away,  the  weakest;  thus  an  evident  dependence 
on  the  main  vein  is  shown.  Moreover,  No.  1  is  strongest  as  it  approaches  its 
junction  with  the  Mizpah.  Here  it  is  at  the  outcrop  composed  of  solid  quartz  4  feet 
wide,  and  appears  to  be  as  important  as  the  Mizpah  itself.  To  the  west,  however, 
the  quantity  of  quartz  in  the  vein  zone  decreases  till  the  vein  is  difficult  to  follow, 
and  very  likely  actually  dies  out.  Vein  No.  2  is  not  regularly  mineralized  and 
has  not  the  characteristics  of  a  strong  fracture  zone.  While  in  general  it  grows 
stronger  on  approaching  the  Mizpah,  the  only  place  from  which  high-class  ore 
has  been  taken  is  several  hundred  feet  west  of  its  junction,  where  the  volume 
of  quartz  in  the  vein  increases.  No.  3  follows  a  definite  fracture  zone  in  the 
andesite  and  ordinarily  has  very  good  walls.  In  this  zone  the  quartz  is  mostly 
in  stringers,  irregular  and  bunchy.  High-grade  ore  was  taken  out  only  from 
one  small  portion  of  the  outcropping  vein,  that  being  opposite  the  productive 
portion  of  No.  2.  The  relation  of  good  walls  to  a  strong  vein  is  continually 
shown.  Good  walls  denote  a  strong  fracture  zone,  which  is  a  good  channel  for 
mineralizing  waters. 

These  veins  have  not  been  found  to  continue  downward  in  general  with  the 
same  strength  that  they  show  on  outcrop,  and  on  the  300-foot  level  of  the  mine 
they  are  represented  only  by  weak  silicifications  or  quartz  seams,  and  not  all  of 
them  are  with  certainty  identifiable. 

VALLEY  VIEW  VEIN  SYSTEM. 
THE   VALLEY   VIEW    VEINS   ON    MIZPAH    HILL. 

The  Valley  View  vein  outcrops,  in  its  strongest  portion,  about  1,000  feet 
south  of  the  Mizpah  vein.  Its  surface  exposures  are  stronger  and  more  compli- 
cated than  those  of  the  Mizpah  system,  showing  a  number  of  veins  which  are 
of  various  sizes,  many  of  them  being  several  feet  thick.  These  are  connected 
by  branches,  so  that  the  whole  is  interlaced.  The  general  course  is  a  little  north 
of  east,  practically  parallel  to  the  Mizpah  vein,  and  the  diiferent  veins  show  a 
tendency  to  fan  out  or  diverge  toward  the  west,  as  the  Mizpah  vein  system  does 
to  a  more  marked  degree.  The  dip  of  the  veins  at  the  surface  is  nearly 
perpendicular,  some  of  them  dipping  north  and  some  south,  at  angles  usually 
approaching  90°. 

CROSS    VEINS    AND    ALLIED    PHENOMENA. 

Cross  veins  of  considerable  strength  also  occur,  both  on  the  east  and  on  the 
west  side  of  the  main  outcrop,  nearly  at  right  angles  to  the  main  course.  These 
cross  veins  cut  off  the  veins  following  the  main  course,  though  all  are  of  the 

16843— No.  42—05 9 


130  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 

same  age.  On  the  east,  the  strong  cross  vein  near  the  Stone  Cabin  shaft  prob- 
ably cuts  off  the  complicated  vein  system  in  this  direction;  beyond  this  cross  vein 
the  other  veins  will  be  found,  for  a  space  at  least,  abruptly  of  a  different 
character.  Similarly  the  strong  northwest-striking  and  northeast-dipping  vein, 
which  heads  off  a  number  of  the  Valley  View  veins  on  the  southwest  side  of 
Mizpah  Hill,  seems  to  mark  the  boundary  of  a  relatively  poorer  continuation  of 
the  main  vein  system  on  the  west  (see  PI.  XVII).  Nevertheless,  some  of  the 
veins  escape  and  persist,  and  are  found  across  the  gulch,  in  outcrops  and  in 
the  workings  of  the  Wandering  Boy.  The  veins  of  the  Fraction  may  be  a  con- 
tinuation of  this  system. 

Apparently  the  mineralizing  solutions  flowing  along  the  east-west  fracture 
zones  were  deflected  where  the  transverse  fracture  zones  were  strong  enough  to 
control  the  circulation,  and  did  not  follow  the  old  channel  farther. 

The  same  principle  is  shown  by  the  numerous  splitting  and  reuniting 
branches,  all  running  in  the  main  direction  of  the  vein  system.  Any  of  these 
branches  may  divert  the  main  strength  of  the  vein  along  it  and  into  a  parallel 
vein  of  the  group. 

This  heading  off  of  the  main  course  of  veins  by  crosscutting  veins  is  entirely 
analogous,  though  on  a  larger  scale,  to  the  crosscutting  premineral  fractures  which 
have  produced  the  cross  walls,  as  studied  out  on  the  Mizpah  vein,  and  so  have 
brought  about  the  localization  of  the  ore  deposits.  The  cross  walls  produce  richer 
shoots,  both  as  regards  quartz  and  precious  metals,  within  the  main  fracture  zone; 
the  cross  veins  cause  relative  differences  in  mineralization  and  vein  formation 
along  portions  of  a  belt  of  interlacing  fracture  zones  which  is  similar  to  though 
larger  than  that  occupied  by  the  main  Mizpah  vein.  In  the  ordinary  splitting 
of  the  veins,  as  seen  in  both  systems,  the  diverging  branches  have  not  so  radically 
different  a  direction  from  the  main  vein  as  have  the  cross  veins,  but  have  often 
operated  to  deflect  the  solutions  from  the  main  fracture  zone,  and  hence  are  called 
vein  robbers. 


VEIN    STRUCTURE    AND    ORIGIN. 


On  studying  the  different  veins  of  the  system,  as  exposed  excellently  in  an 
almost  continuous  series  of  surface  openings,  the  fact  that  these  veins  are  due  to 
the  replacement,  in  varying  degrees,  of  andesite  by  quartz  along  a  zone  of  especial 
fracturing  is  well  illustrated.  This  is  shown  by  the  ever-changing  amount  of 
replacement,  at  one  point  the  vein  zone  being  little  more  than  fractured  porphyry 
and  at  another  solid  quartz,  with  all  conceivable  transitional  stages  represented 
between,  these  points  are  illustrated  by  the  sections  forming  fig.  28. 


VALLEY    VIEW    VEIN    SYSTEM. 


131 


Vein  zone 


Vein  zone 


Quartz  veinlcts    Quartz    Quartz  veinfets 


Scale 


s        10  feet 


FIG.  28. — Sections  showing  the  structure  of  the  Valley  View  veins;  o,  altered  andesite;  6,  quartz.  A,  Detailed  vertical 
section  of  one  of  the  minor  Valley  View  veins  at  surface.  B,  Detailed  vertical  cross  section  of  the  same  vein  as 
A,  taken  about  30  feet  east  of  it.  C,  Detailed  sketched  cross  section  of  the  same  vein  as  A  and  B  taken  about  70 
feet  east  of  S.  D,  Vertical  sketched  cross  section  of  the  same  vein  as  A,  B,  and  C,  at  the  surface,  taken  at  a  point  about 
50  feet  east  of  C.  E,  Detailed  sketched  vertical  cross  section  of  a  fracture  zone  near  Valley  View  shaft,  at  surface, 
showing  the  nature  of  the  fracture  zone,  which  by  replacement  may  form  a  solid  quartz  vein.  Even  the  small  stringers 
shown  here  have  unquestionably  originated  by  replacement  of  the  andesite  along  mere  cracks. 


132  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 


ORE    IX    TIIK    VKIX. 


As  a  rule  the  Valley  View  vein  system  contains  a  larger  volume  of  vein 
material  than  the  Mizpah  system,  but  a  smaller  amount  of  the  precious  metals. 
Therefore  the  Mizpah  has  produced  more  ore  than  the  larger  vein.  Considerable 
ore  of  the  kind  locally  considered  low  grade  (up  to  $50  per  ton,  for  example)  has 
been  found  in  the  Valley  View  veins,  and  some  other  portions  have  been  found 
to  be  very  rich:  this  rich  ore  lies  in  masses,  without,  so  far  as  yet  developed,  any 
regular  extension. 

THE   VAI-LEY   VIEW   VEIN    SYSTEM    UNDERGROUND. 

Underground  on  the  Valley  View  vein  system  are  the  workings  from  the  main 
Valley  View  shaft,  those  from  the  near-by  Silver  Top  shaft  (both  of  these  shafts 
belong  to  the  Tonopah  Mining  Company),  and  those  of  the  Stone  Cabin  shaft, 
and,  as  before  stated,  outside  of  Mizpah  Hill,  probably  the  Wandering  Boy  and 
Fraction  workings. 


VK1XS    IN    THE    VALLKY    VIEW    WORKINGS. 


Of  these  underground  workings  those  of  the  Valley  View  are  the  most 
extensive.  There  were  levels  at  depths  of  200,  300,  400,  and  500  feet  at  the  time 
of  the  writer's  last  visit.  Instead  of  the  several  parallel  strong  veins  outcropping 
at  the  surface,  these  workings  show  a  single  main  vein,  which  is  thicker  than 
any  at  the  surface.  Furthermore,  while  the  surface  veins  are  nearly  perpen- 
dicular, the  underground  vein  has  a  dip  to  the  north  of  less  than  45°.  This 
vein  is  tj  or  8  feet  or  more  thick  in  various  places. 

Other  veins  disclosed  in  the  workings  are  weak  and  nonpersistent,  though 
locally  they  may  be  a  few  feet  thick  and  may  hold  out  promising  indications. 
Frequent  quartz  stringers,  which  may  be  so  numerous  in  places  as  to  form  nearly 
a  network,  occur;  and  plainly,  from  what  is  known  of  the  general  geology,  these 
scattered  threads  might  at  any  point  unite  vertically  or  laterally  and  form  a 
decided  vein,  and  thus  account  for  the  veins  which  outcrop  and  are  not  cut  in 
underground  workings,  or  those  encountered  in  one  mine  level  and  not  in  the 
expected  place  in  another.  Many  of  these  stringers  are  vertical,  so  that  they 
would  merge  with  the  flatter-lying  main  vein  in  a  short  distance.  The  general 
situation  would  seem  to  be  represented  in  rig.  2!».  A  strong  east-west  striking 
and  north-dipping  vein  (associated  with  parallel  and  crosscutting  minor  veins  and 
stringers,  many  of  them  nearly  vertical)  has  given  its  strength  near  the  present 
surface  to  a  number  of  vertical  transverse  fractures,  so  that  the  main  vein  splits 
into  a  number  of  vertical  ones. 

On  the  oOo-foot  level  the  vein  is  still  strong.  The  crosscut  on  the  700-foot 
level,  however,  did  not  encounter  it,  but  passed  through  a  body  of  white,  dense 


VALLEY    VIEW   VEINS   UNDERGROUND. 


133 


rock,  which  microscopic  study  showed  to  be  probably  the  Tonopah  rhyolite-dacite. 
This  rock  is  a  part  of  an  intruded  sheet,  which  would  cut  off  the  vein  and  termi- 
nate it  below,  at  least  temporarily  (tig.  30). 


sw. 


NE. 


'!:'  \^:>:^-'l^f':':  ^'feK  '~-':^^T^<:  ~i''[-  -",-'',   '.\''     '   '  ','-  •<;'.'::s- 


:;300-ft.  level ;/7 


B 

Scale 


100  200  feet 


Fie.  29. — CroKsjieotions  of  the  Valley  View  vein.    A,  Through  the  Valley  View  shaft.    B,  Cross  section  of  Valley  View  rein 
taken  a  short  distance  (averaging  200  feet)  west  of  section  A. 

THE    VALLEY     VIEW    FAULT. 

Postmineral  fractures  are  abundant  in  the  mine  workings,  and  u  notable  fault 
occurs  on  the  200-foot  level,  by  which  the  main  vein  is  completely  cut  off  and 
lost  on  the  east  side.  This  fault  has  here  a  strike  of  N.  15°  E.  and  a  dip  of 


134 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 


50°  E.  On  the  300-foot  level  east  the  vein  is  likewise  cut  off  by  a  broken  and 
fractured  zone,  with  finally  a  .straight  slip  face  running  N.  28°  E.  and  dipping 
steeply  east.  These  occurrences  on  the  two  levels  represent  probably  the  same 
general  fault  or  fault  zone.  This  fault,  which  may  be  called  the  Valley  View 
fault,  is  therefore  approximately  parallel  in  strike  and  dip  with  the  Stone  Cabin 


Valley  View  shaft 


LATER  ANDESITE      Siebert  shaft 


Flo.  30.— Vertical  section  on  plane  of  Siebert  and  Valley  View  shafts. 

fault,  which  bounds,  on  the  east  side,  the  earlier  andesite  of  Mizpah  Hill,  and 
separates  it  at  the  surface  from  the  tuff  formation  (Siebert  lake  beds)  on  the  west. 
The  Stone  Cabin  and  the  Silver  Top  workings,  therefore,  are  on  the  east  side 
of  the  Valley  View  fault,  or  between  the  Valley  View  and  the  Stone  Cabin  faults, 
while  the  Valley  View  workings  are  on  the  west  side  of  the  fault  of  the  same  name. 


VALLEY    VIEW    VEIN    SYSTEM. 


VEINS    IN    THE    STONE    CABIN    WORKINGS. 


135 


The  Stone  Cabin  shaft  had  followed  a  strong  vein  from  near  the  surface  to  a 
depth  of  400  feet  at  the  time  of  the  writer's  last  examination  (fig.  31).  This  vein 
varies  in  thickness  from  1  to  8  feet,  averaging  perhaps  3  feet.  It  strikes  N. 
450  to  50°  E.,  and  is  evidently  one  of  the  main  east-  west  veins  of  the  Valley 
View  system,  which  here  swings  around  more  to  the  north.  From  the  surface  to 
the  200-foot  level  it  dips  steeply  to  the  southeast,  and  thence  to  the  400-foot  level 
it  is  vertical. 

About  30  feet  east  of  it  a  parallel  vein  is  encountered  on  the  100-foot  level, 
but  it  is  so  much  broken  up  by  small  faults  that  it  can  not  easily  be  followed. 
These  faults  strike  chiefly  N.  25°  to  '40°  W.,  and  dip  southeast  at  an  angle  of 


FIG.  31. — Cross  section  of  veins  in  Stone  Cabin  workings. 

55°.  They  are  probably  auxiliary  to  the  main  Stone  Cabin  fault,  which  must 
be  close  at  hand,  judging  from  its  position  in  the  near-by  Silver  Top  workings. 
On  the  200-foot  level  the  same  vein  is  encountered,  at  the  same  relative  position 
with  regard  to  the  main  vein.  Here,  being  farther  away  from  the  eastward- 
dipping  fault,  it  is  not  broken.  On  the  other  hand,  it  is  not  so  heavy  as  above, 
and  consists  of  two  diverging  branches,  each  1  foot  thick,  which  unite  in  the  bottom 
of  the  crosscut.  On  the  400-foot  level,  200  feet  below,  this  vein  was  not 
recognized. 

Thus  there  is  a   single   nearly  vertical   strong  vein  in  the  Stone   Cabin,  as 
in   the   Valley   View   workings,  with   another   lesser  vein  parallel   to   it  on   the 


136 


GEOLOGY    OK    TONOPAH    MINING    DISTRICT,   NEVADA. 


east,  which  seems  to  grow  weaker  and  tends    to  disappear    in  depth.     The   main 
workings  have  been  driven  on  the  first  vein. 

Much  of  the  ore  is  of  low  grade,  and  not  much  runs  over  $100  to  the  ton. 
A  considerable  quantity  of  moderate  grade  ore  has  been  found.  This  ore  lies 
largely  in  an  ore  shoot,  which  pitches  steeply  east  on  the  vein,  and  which 
was  followed  from  near  the  surface  to  below  the  400-foot  level. 


:V'-.'f:'/->T<?P  of  Adrift •>.•/;•  /^j.-'.^ 


. 

?®Wj^i 

^.:--.;-'-.----: 


• • "       ^' .        .  X       1  •  ^  *     '       -     •       • 

^•.r.';^^  •  y>;- '~'-i *--\ 


•,:i-  ./•.-'•••<:1-  !••:.•'••-'' •-. '• 
••^.^.•<v.-.v -••>•.•.•>:  i 

iv-'/'-'j-'/'  ~ :  ^  .Vi;H;v.i, 


Fie.  32.— Sketch  of  vertical  cut  on  the  east  wall  of  the  Silver  Top  120-foot  level,  3  feet  south  of  mnlii  vein,  showing 

splitting  and  reuniting  of  a  minor  vein. 

VEINS    IN    THE    SILVER   TOP    WOKKINOB. 

The  Silver  Top  shaft  of  the  Tonopah  Mining  Company  "  starts  on  the  oast  side 
of  the  Stone  Cabin  fault,  in  finely  bedded  white  tuff,  striking  N.  20-  W.  and 
dipping  southwest.  Below  this  is  the  later  andesite  to  the  bottom  of  the  shaft, 

•  There  U  another  Silver  Top  shaft  northeast  of  the  Golden  Anchor,  as  shown  In  the  general  r.iap. 


VALLKY     VIEW    VEIN    SYHTEM.  137 

which  is  120  feet  deep.  An  easterly  drift  cuts  the  fault  and  passes  into  the  earlier 
andesite.  On  this  drift  are  found  the  veins  encountered  in  the  Stone  Cabin 
workings  (tig.  32).  The  chief  vein  here  seems  to  run  N.  50°  to  70°  E.  and  to 
dip  south  at  an  angle  of  about  80°.  It  is  encountered,  though  in  a  broken-up 
condition,  just  west  of  the  fault,  but  it  is  undoubtedly  cut  off  by  this  on  the  east. 
To  the  southwest  of  this  fault  the  vein  lies  in  the  Stone  Cabin  ground,  and  has 
been  developed  by  the  workings  on  the  100-foot  level  of  this  mine  for  about  100 
feet.  Still  farther  southwest  the  vein  comes  again  into  the  Silver  Top  ground, 
and  is  followed  southwest  from  the  Stone  Cabin  ground  for  about  140  feet.  At 
somewhat  over  100  feet  southwest  of  the  Stone  Cabin  ground  the  vein  forks, 
and  at  the  end  of  the  drifts  both  forks  are  cut  off  by  a  fault  striking  N.  22° 
W.  and  dipping  eastward  at  an  angle  of  60°.  The  vein  is  also  developed  by  a 
vertical  winze  HO  feet  in  depth  in  the  portion  west  of  the  Stone  Cabin  ground. 

THE   STONE   CABIN-SILVER   TOP    VEINS    A    PART   OF  THE    VALLEY    VIEW    VEIN    OROl'P. 

The  second  weaker  and  parallel  vein  noted  in  the  Stone  Cabin  workings,  to 
the  south  of  the  main  vein,  appears  also  in  the  Silver  Top  workings,  but  has  not 
been  developed. 

The  probable  equivalents  of  both  of  these  veins,  which  are  shown  in  the 
Silver  Top  and  Stone  Cabin  workings,  can  be  recognized  on  the  surface,  almost 
immediately  above,  at  the  east  end  of  the  outcrop  of  the  Valley  View  veins, 
where  they  have  the  same  characteristics  that  are  given  for  the  corresponding 
veins  underground.  Even  the  forking  of  the  vein  in  the  Silver  Top  west  drift, 
as  described  above,  corresponds  with  a  similar  forking  of  the  corresponding  vein 
at  the  surface. 

It  is  plain,  then,  that  the  veins  in  the  Stone  Cabin  and  the  Silver  Top  belong 
to  the  Valley  View  group;  and  that,  as  in  the  case  of  the  Valley  View  mine, 
this  outcropping  group  resolves  itself  underground  into  a  single  strong  and 
persistent  vein  with  parallel  weaker  veins. 

CORRELATION   OF   VEINS   IN    DIFFERENT   MINES. 

If  this  is  the  case,  why  does  the  vein  go  down  nearly  vertically  for  a  known 
distance  of  450  feet  in  the  Stone  Cabin  and  Silver  Top,  while  in  the  Valley 
View  the  vein  dips  north  at  an  angle  less  than  45°  for  a  known  vertical  depth 
of  500  feet? 

Effects  of  the  Valley  View  fault. — In  approaching  this  problem  we  confront 
first  the  fact  that  underground  the  veins  of  the  Stone  Cabin  and  Silver  Top 
have  not  been  followed  westward  beyond  a  certain  point,  and  that  the  Valley  View 
has  not  been  traced  eastward  beyond  a  certain  point.  The  veins  are  clearly  cut  off 


138 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 


by  strong  faults  that  strike  north  and  dip  east.  Strangely  enough  these  faults 
have  not  been  recognized  on  the  surface,  at  the  points  where  they  should 
outcrop,  but  of  their  importance  underground  there  is  no  doubt. 

The  slip  and  cut-off  at  the  west  end  of  the  Silver  Top  workings  must  be  only 
about  70  feet  perpendicularly  distant  from  the  slip  which  cuts  off  the  Valley  View 
vein  on  the  east,  on  the  200-foot  level.  The  two  can  be  treated  together,  then,  as 
a  fault  zone,  in  which  the  main  slip  may  or  may  not  have  been  cut,  but  must  either 
be  one  of  those  cut  or  must  lie  in  the  narrow  zone  between  them.  Moreover, 
the  flat-dipping  Valley  View  vein,  which  is  thus  cut  off  on  the  200-foot  level. 


Valley  View  shaft 


Store  Cabin  shaft 
8O°I 


ZOO-ft. level,  7  feet  below 
Valley  View  ZOOft.level 


50 


200  feet 


FIG.  33.— Horizontal  plan  of  veins  in  Valley  View  and  Stone  Cabin  workings  on  the  plane  of  the  Mizpah  200-foot  level, 
to  show  the  probable  connection  between  the  chief  veins  on  the  two  sides  of  the  Valley  View  fault. 

would,  if  continued,  almost  exactly  strike,  at  this  level,  the  nearly  vertical  main 
Silver  Top  ledge,  which  trends  in  the  same  direction  (fig.  33). 

Hypotheses  to  explain  fault  movement. — The  suggestion  arises  that  the  Silver 
Top  and  the  Valley  View  may  be  really  parts  of  the  same  vein,  and  that  faulting 
is  responsible  for  the  remarkable  differences  in  dip  on  the  two  sides  of  the 
fault.  Both  are  plainly  the  downward  extension  of  the  strongest  portion  of  the 
same  outcropping  vein  system.  We  may  at  first  consider  the  hypothesis  whether 
the  fault  has  had  a  twisting  movement  so  as  to  tilt  the  vein  on  one  side  more 
than  on  the  other.  This  difference  in  tilt,  however,  would  be  about  45°,  and 
would  involve  such  an  extraordinary  rotation  of  the  rocks  on  one  side  of  the 


VALLEY    VIEW    VEIN    SYSTEM.  139 

fault  that  the  truth  of  the  hypothesis  may  well  be  doubted.  Another  possible 
hypothesis  may  be  formulated.  Comparison  of  the  vein  in  the  main  Valley  View 
workings  and  in  the  outcrops  shows  that  near  the  surface  the  strong  north- 
dipping  vein  underground  changes  by  branching  into  a  number  of  vertical  veins, 
which  are  strong,  yet  not  so  strong  as  the  main  veins.  In  the  Stone  Cabin  and 
Silver  Top  workings  these  vertical  veins  extend  far  deeper  than  on  the  Valley 
View  side  of  the  fault  and  no  flat  vein  has  been  encountered.  It  follows  as  a 
satisfactory  explanation  that  the  veins  on  the  east  have  been  dropped  down  by 
the  fault  vertically,  so  that  the  upper  vertical  portions  come  opposite  the  lower 
flat  portion  on  the  west. 

It  is  not  easy  of  explanation  on  either  hypothesis  why  the  fault  has  not  been 
recognized  on  the  surface.  Especially  under  the  hypothesis  of  rotation  or  differential 
tilting  is  this  fact  inexplicable,  for  such  rotation  must  have  been  likewise  manifested 
at  the  surface  as  a  great  and  sustained  difference  in  the  vein  dips;  whereas  actually 
no  such  change  occurs,  the  steep,  nearly  vertical,  dip  being  unvarying  over  the  area 
which  the  fault  would  naturally  cut.  If,  however,  according  to  the  second  hypothesis 
of  simple  downward  displacement,  the  movement  is  assumed  to  have  been  absolutely- 
vertical,  there  is  at  least  a  possible  explanation  of  the  failure  to  detect  the  fault — 
namely,  that  in  the  surface  portion  of  the  group  the  vertical  veins,  broken  by  a 
vertical  fault,  would  not  show  any  displacement,  while  below,  where  the.  vertical 
veins  come  opposite  the  flat  ones,  the  displacement  would  be  marked. 

The  probability  of  this  latter  hypothesis  is  strengthened  by  a  consideration  of 
the  main  Stone  Cabin  fault,  which  has  a  general  parallelism  in  strike  and  dip  with 
the  Valley  View  fault,  and  lies  about  250  feet  horizontally  east  of  it.  This  fault  is 
a  normal  one,  having  a  heavy  downthrow  on  the  east  side,  bringing  the  tuffs,  and 
below  these  the  later  andesites,  opposite  the  earlier  andesite  of  Mizpah  Hill  on  the 
west.  It  is  probable  that  a  near-by  parallel  fault,  like  the  Valley  View  fault,  would 
have  a  movement  in  the  same  direction.  The  Valley  View  fault  is  evidently  much 
the  smaller  of  the  two,  and  may,  indeed,  be  considered  auxiliary  to  the  main 
displacement. 

Amount  of  vertical  separation  of  Valley  View  fault. — The  amount  of  vertical 
movement  at  the  Valley  View  fault,  on  the  basis  reached  above,  would  be  something 
over  400  feet.  This  affords  some  basis  for  understanding  the  movement  on  the 
greater  Stone  Cabin  fault,  which  may  reasonably  be  expected  to  be  several  times 
greater. 


140  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,  NEVADA. 

FRACTION    NO.    1    VEINS. 

DISCOVERY    AND   DEVELOPMENT. 

The  No.  1  Fraction  shaft  was  sunk  blindly  in  the  outcropping  dacite  of  the 
Fraction  dacite  breccia  in  the  fall  of  1901,  and  was  one  of  the  first  explorations 
outside  of  Mizpah  Hill  and  Gold  Hill.  The  shaft  was  sunk  to  the  depth  of  238 
feet  by  means  of  a  horse  whim.  The  shaft  passed  through  150  feet  of  soft  dacite, 
20  feet  of  crushed  material  probably  representing  the  later  andesite,  several  feet 
of  breccia  indicating  a  probable  fault  zone,  and  ended  in  the  earlier  andesite.  At 
the  depth  of  238  feet,  the  rope  not  being  long  enough  to  sink  any  farther,  drifting 
was  started,  which,  in  20  feet,  cut  a  body  of  quartz  that  had  a  width  of  several 
feet  and  showed  some  rich  ore.  Subsequent  to  this  a  great  deal  of  development 
work  has  been  done,  but  the  results  have  been  unsatisfactory,  the  vein  being  very 
badly  faulted  and  there  being  very  little  rich  ore. 

NATURE   AND   RELATIONS   OF  THE    FRACTION    VEIN. 

By  looking  at  the  detailed  map  of  the  mining  district  it  will  be  seen  that 
the  Fraction  workings  lie  close  to  two  faults  which  were  drawn  from  surface 
indications.  A  study  of  the  underground  workings  indicates  that  the  faulting  has 
been  so  intense  and  complicated  as  to  defj'  working  out  of  the  smaller  details  and 
as  to  make  the  mining  under  these  conditions  practically  hopeless,  unless  the  ore 
were  very  rich. 

Apparently  a  single  strong  vein  is  represented  in  the  Fraction  workings.  This 
strikes  in  general  east  and  west,  but  frequently  north  of  west,  and  dips  south  at 
varying  angles.  This  vein  is  in  line  with  the  outcrops  of  the  Valley  View  veins 
across  the  gulch  to  the  east  on  Mizpah  Hill.  It  is  possible,  therefore,  that  it 
belongs  to  the  Valley  View  system.  On  the  other  hand,  the  Valley  View  and 
the  Fraction  veins  underground,  when  plotted  on  a  given  level,  show  no  signs  of 
being  part  of  the  same  body,  following  quite  different  lines.  Indeed,  the  two 
veins  dip  in  opposite  directions — the  Valley  View  to  the  north,  the  Fraction  to  the 
south.  The  two  are  also  separated,  as  shown  on  the  map,  by  one  or  more  faults 
which  makes  correlation  still  more  doubtful.  If  the  Fraction  is  part  of  the  Valley 
View  svstem,  its  vein,  dipping  in  the  opposite  direction,  might  be  considered  as 
making  up  with  the  Valley  View  vein  a  pair  of  conjugated  veins.  It  is  barely 
possible,  though  perhaps  not  probable,  that  the  fault  movement  has  in  the  case 
of  the  Fraction  reversed  the  original  dip  by  tilting  the  block  in  which  the  vein  lies. 

Some  of  the  numerous  faults  which  cut  the  vein  have  been  exposed  in  the 
mine  workings,  and  such  have  been  shown  in  the  accompanying  detailed  plans 
and  cross  section  (PI.  XXI). 


FRACTION    NO.   1    VEINS. 


THE    NORTHEAST    ( FRACTION )    FAULT    SYSTEM. 


141 


When  the  strikes  of  all  the  different  faults  observed  in  the  workings  are 
plotted  together,  as  in  tig.  34,  they  are  seen  to  run  in  almost  every  direction 
without  any  fairly  recognizable  sj^stem.  Considered  as  to  their  relative  impor- 
tance, however,  systems  are  clearly  traceable.  The  most  important  one  is,  perhaps, 
that  .striking  in  a  general  northeast  direction  and  dipping,  as  a  rule,  southeast 
at  varying  angles,  perhaps  approximating  45°.  By  these  faults  the  vein,  as  seen 
on  a  horizontal  plan,  is  moved  to  the  north  on  the  west  side.  There  are  many  of 
these,  which  distribute  the  faulting  between  them  and  constitute  a  fault  zone 


Km.  34.— Plotting  of  tlie  strike  of  the  faults  In  tin?  Fraction  workings. 

whose  limits  and  total  displacement  are  not  known.  If  this  fault  zone  had  a 
uniform  dip  it  would  reach  the  surface  about  where  the  fault  line  had  been 
independently  drawn,  from  surface  phenomena,  up  the  gulch  on  the  southeast  side 
of  Brougher  Mountain.  This  fault  line,  as  will  be  seen,  seems  to  be  a  direct 
continuation  of  that  bounding  the  earlier  andesite  of  Mizpah  Hill  on  the  northeast, 
but  the  fault  movements  probably  do  not  correspond  in  the  two  localities. 

On  the  237-  and  300-foot  levels  of  the  Fraction  this  northeast  faulting  has 
divided  the  vein  into  a  series  of  blocks  of  very  limited  extent  horizontally,  which 
have  l>een  dragged  apart  and  separated  one  from  another,  and,  finally,  the  vein 
has  t>een  lost  on  account  of  these  faults,  both  on  the  east  and  on  the  west  side. 


142 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 


On  the  237-foot  level  there  is  a  distance  of  about  200  feet  between  the  portion 
of  the  vein  just  north  of  and  that  south  of  the  shaft,  as  exposed  in  the  drifts, 
but  connecting  bunches  of  quartz  probably  exist  in  the  undeveloped  country 
to  the  southeast  of  the  shaft  (fig.  35).  On 
the  300-foot  level  the  exploration  has  beer 


Fraction  No. I 
shaft 


\ 


'I 

\     \ 


\      > 

\ 

\\ 
\\ 

Scale 
o      10     20     30     40     so  feet 


FraaionNoJ  sh 


Via.  36. — Horizontal  plan  of  vein  aud  limits  on  ine  '£>i 
foot  level,  Fraction  No.  1  workings. 


Fio.  36.— Horizontal  plan  showing  vein  and  faults  on 
the  800-foot  level.  Fraction  No.  1  workings. 


more  thorough,  as  far  as  it  went,  and  the  different  steps  of  the  faulting  are  almost 
continuously  shown  (fig.  36).  On  the  400-foot  level  a  single  block  of  quart/,  probably 
belonging  to  the  same  vein,  and  bounded  on  all  sides  by  faults,  was  found  about 


FRACTION    NO.    1    VEINS. 


143 


300  feet  south  of  the  shaft  (fig.  37).  On  account  of  the  eastward  dip  of  the 
northeast-striking  fault  zone  this  bunch  of  quartz  would  lie  to  the  west  of  the  fault 
and  would  correspond  in  position  to  the  quartz  near  the  shaft  on  the  two  upper 


FIG.  37.— Horizon uil  plan  showing  veins  and  faults  on  the  400-foot  level.  Fraction  No.  1  workings. 

levels.  This  is  shown  also  by  the  connection  made  between  the  300-  and  the  400-foot 
levels,  where  the  relations  of  the  vein  on  the  west  of  the  northeast-striking  fault 
zone  are  as  shown  in  the  accompanying  cross  section  (fig.  39). 


144 


GEOLOGY    OF   TONOPAH    MINING    DISTRICT,    NEVADA. 


Abundant  and  strong  striations  on  the  fault  planes  of  the  northeast  system, 
together  with  the  evidence  afforded  by  minute  faulting  and  stringers  and  b3' 
the  dragging  of  faulted  veins,  indicate  that  while  the  main  movement  was  com- 
plicated by  numerous  smaller  ones,  the  general  result  was  that  the  blocks  on  the 
west  side  of  the  separate  northeast  faults  were  shoved  northward  past  the  blocks 
on  the  east  side,  nearly  horizontally,  but  with  a  slight  downward  plunge  (tig.  38). 


Fid.  3x.— Stereogram  showing  nature  of  movement  along  the  main  northeast  faults  in  Fraction  No.  1  workings. 

THE    NORTHWENT    FAl'l.TS. 

There  is  also  a  well-marked  system  of  faults  striking  north  of  west,  sometimes 
parallel  to  the  veins,  but  generally  cutting  across  them  at  slight  angles.  These 
faults  may  have  some  connection  with  the  northwest  fault,  which  is  shown  on  the 
map  as  running  just  east  of  the  Fraction  workings  (the  Wandering  Boy  fault). 
Thev  have  a  great  variety  of  dips,  sometimes  vertical  and  sometimes  nearly 
horizontal,  with  intermediate  angles.  An  illustration  of  their  effects  is  seen  in 


FRACTION    NO.   1    VEINS. 


145 


the  cross  section  (fig.  39),  which  is  transverse  both  to  the  vein  and  to  the  faults. 
.This  crass  section  is  taken  along  the  series  of  inclined  workings  on  the  vein, 
which  run  from  a  point  about  60  feet  above  the  upper  level  to  below  the  lower 
level.  The  portions  actually  exposed  are  indicated  by  solid  lines,  the  intervening 
portions  are  dotted.  It  will  be  seen  that  the  vein  follows  a  series  of  pronounced 
rolls,  steepening  and  flattening  alternately.  In  the  mine  it  is  evident  that  these 
rolls  are  the  result  of  pressure  and  deformation  in  the  rock,  and  are  in  the 
nature  of  folds.  On  the  two  upper  levels,  at  the  sharp  bend  or  apex  of  these 
folds,  as  shown  in  the  cross  section,  tangential  fractures  or  slight  faults  leave  the 


Scale 

30 


mofeet 


300-rt./evel 


400-ft./e*el 


FIG.  39. — Cross  section  of  Fraction  No.  1  vein,  along  drifts  and  winzes. 

vein  and  pass  off  into  the  surrounding  andesite.  Some  of  these  become  horizontal, 
some  nearly  vertical,  and  both  strike  nearly  parallel  with  the  vein.  Between  the 
300-  and  the  400-foot  levels,  a  flat  fault,  striking  and  dipping  in  the  same  way  as  the 
vein,  has  probabty  the  same  origin  as  the  flat  tangential  slips  in  the  upper  levels, 
but  is  here  of  greater  magnitude,  so  that  the  vein  has  actually  been  faulted  con- 
siderably along  it.  The  incline  from  the  300-  to  the  400-foot  level  follows  this 
fault  for  some  distance  after  the  disappearance  of  the  vein.  The  fault  which 
terminates  the  vein  at  its  lower  end  in  the  cross  section  belongs  to  the  northeast 
system,  and  Is  thus  different  from  any  other  of  the  faults  shown  in  the  figure. 
16843— No.  42—05 10 


146  GEOLOGY-  OF    TONOPAH    MINING    DISTRICT,   NEVADA. 

The  deformation  displayed  in  this  section  of  the  vein  is  analogous  to  the 
monoclinal  folding  of  strata,  in  which  the  fold  passes  into  a  fault  if  the  deforma- 
tion be  carried  farther  than  the  stretching  strength  of  the  rocks.  Since  all  the 
veins  in  the  Tonopah  district  have  normally  decided  dips,  ranging  from  vertical  to 
about  30°,  it  may  be  believed  that  the  flatter-dipping  portions  of  the  Fraction 
vein  as  seen  in  the  cross  section  have  been  deformed,  and  that  the  steeper  portions 
represent  more  nearly  the  original  attitude.  It  appears  then  that  the  vein  has 
been  deformed  by  movements  acting  in  a  nearly  horizontal  plane.  These 
movements  have  shoved  the  vein  and  the  inclosing  country  rock  to  the  north  on 
the  upper  side,  and  being  distributed  have  caused  rolls  or  folds  which  in  places 
break  and  form  faults. 

These  west-northwest  tangential  faults  are,  however,  not  persistent^  parallel 
to  the  veins,  but  may  trend  across  them  at  a  slight  angle.  The  result  is  seen  in 
the  western  part  of  the  workings  on  the  237-  and  300-foot  levels,  as  shown  in 
the  figures.  On  the  300-foot  level  the  west-northwest  fault  cuts  out  the  vein 
gradually.  The  vein  runs  parallel  with  the  fault  for  some  distance,  appearing 
and  reappearing  as  lenses  of  quartz  along  the  fault  zone,  until  it  entirely 
disappears. 

CAUSE  OP   FAULTING. 

To  explain  this  singularly  intense,  complicated,  and  peculiar  faulting  there 
must  be  found  a  cause  competent  to  thrust  the  blocks  on  the  northwest  side 
of  the  northeast  faults  to  the  north  in  a  nearly  horizontal  direction,  and. 
to  shove  the  upper  layers  of  rock  and  vein  past  the  lower  layers  in  a  nearly 
horizontal  direction  also.  The  volcanic  neck  of  Brougher  Mountain,  whose 
edge  is  only  about  1,400  feet  southwest  of  the  Fraction  No.  1  workings,  has  been 
thrust  up  after  the  other  rocks  were  erupted  and  the  mineral-bearing  veins 
formed.  Its  smallest  diameter  in  a  north-south  direction  is  about  1,200  feet,  and 
the  examination  of  its  contact  zone  shows  that  it  probably  extends  downward  in 
much  the  same  form  as  it  appears  at  the  surface,  as  a  solid  column  of  lava. 
The  intrusion  of  this  column  was  probably  competent  to  produce  this  complicated 
faulting,  and  to  exert  the  violent  horizontal  pressure  indicated  in  the  Fraction 
workings.  It  has  been  determined  independently  that  the  faults  of  the  region, 
as  a  whole,  came  into  existence  at  about  the  period  of  the  intrusion  of  the 
dacitic  necks,  of  which  Brougher  Mountain  is  one.  The  conclusion  arrived  at 
therefore  falls  in  line  with  the  general  facts. 

COMPOSITION   OF  VEIN. 

A  small  quantity  of  rich  ore  was  taken  from  the  upper  levels.  This  ore 
showed  ruby  silver  and  argentite  and  in  one  case  native  silver,  all  in  leaves  or 
films  on  cracks  or  crevices,  evidently  secondary.  The  rich  quartz  itself,  as  in 


FRACTION    NO.   2    WORKINGS. 


147 


other  mines  of  the  district,  has  a  dull  purplish  color,  due  to  the  presence  of  fine 
silver  sulphide.  Most  of  the  quartz  discovered,  however,  has  proved  to  be  of  low 
grade.  Adularia  (valencianite)  is  very  abundant  as  a  gangue  material. 

FRACTION   NO.  2    WORKINGS. 


ROCKS    EXPOSED    IX    SHAFT. 


The  Fraction  No.  2  shaft,  which  was  sunk  after  the  No.  1  shaft  and  became 
the  main  working  shaft,  lies  about  450  feet  west-southwest  of  the  No.  1  shaft  and 
is  connected  with  it  at  the  400-foot  level.  The  collar  is  slightly  higher  than  that 
of  the  No.  1  shaft,  and  the  geologic  section  exposed  is  about  the  same.  The  shaft 
passed  through  about  215  feet  of  soft  dacite  and  about  8  feet  of  white  breccia 
(consisting  in  large  part  of  rhyolite  resembling  the  Oddie  rhyolite)  into  the 
earlier  andesite.  The  contact  of  the  overlying  rocks  with  the  earlier  andesite 
dips  to  the  east  at  an  angle  of  about  30°,  but  this  dip  is  probably  only  local. 


Scale 


Fraction  No.2 shaft 
300-ft.  level 


FIG.  40.— Horizontal  plan  of  veins  and  faults  exposed  on  the  300-foot  level,  Fraction  workings,  showing  the  relation  of 
the  vein  fragment  in  the  Fraction  No.  2  to  the  vein  on  the  corresponding  level  of  Fraction  No.  1. 

FAULTED  VEIN   FRAGMENT. 

At  about  300  feet  from  the  shaft  a  body  of  quartz  was  drifted  on  for  a  short 
distance  to  the  southwest.  This  quartz  is  a  definite  vein  about  3  feet  thick.  It 
strikes  northeast  and  dips  southeast  at  an  angle  of  about  40°.  Some  good  assays 
were  obtained  from  it,  although  most  of  it  was  very  low  grade.  On  the  north- 
west side  of  the  shaft  this  vein  seems  to  be  cut  off  by  a  flat  fault  that  strikes  a 
little  west  of  north  and  dips  at  a  slight  angle  to  the  east.  It  is  very  likely  that 
this  vein,  which  has  not  been  very  largely  explored,  may  be  part  of  the  same 
vein  which  is  exposed  in  the  Fraction  No.  1  workings,  although  a  plotting  of  the 
vein  on  the  corresponding  levels  in  the  No.  1  and  No.  2  workings  shows  how 
difficult  it  is  to  establish  any  definite  connection  (fig.  40).  Only  the  size  and 


148  GEOLOGY   OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

nature  of  the  veins  and  the  corresponding  strike  and  dip  warrant  the  above  sug- 
gestion, for  the  faulting  is  so  complicated  in  this  region  that  in  any  space  actually 
undeveloped  by  mining  operations,  little  more  than  guesses  can  be  made  in  many 
cases. 

TONOPAH    RHYOLITE-DAC1TE. 

The  Fraction  No.  2  workings  lie  mostly  on  the  400-foot  level,  and  besides  a 
connection  with  the  No.  1  shaft  there  is  a  drift  running  nearly  600  feet  to  the 
north-northwest  and  more  than  200  feet  in  the  opposite  direction.  Only  small 
quartz  veins,  of  no  importance,  occur  in  these  workings.  The  rock  encountered 
is  a  rather  dark-colored  earlier  andesite,  sometimes  considerably  kaolinized,  like 
that  encountered  in  the  No.  1  workings.  In  the  south  drift  from  the  shaft, 
however,  a  white  rock  is  encountered.  This  is  solid  at  the  end  of  the  south  drift, 
and  Ijetween  this  point  and  the  shaft  occurs  as  fragments  and  large  bowlders  up 
to  several  feet  in  diameter  in  the  darker  andesite.  The  geologic  features  here 
indicate  that  the  breccia  is  due  to  movement  in  the  rock,  and  this  conclusion  is 
corrroborated  by  microscopic  study.  In  this  breccia  are  encountered  several 
strongly  marked  slip  planes,  which  strike  N.  30°  or  40°  E.,  and  dip  southeast  at  an 
angle  of  40°  or  more.  These  correspond  in  altitude  to  the  northeast-striking  and 
southeast-dipping  faults  in  the  Fraction  No.  1  workings,  and  it  appears  probable 
that  the  hard  white  rock  at  the  south  end  of  the  drift  has  been  brought  against 
the  darker  and  softer  andesite  of  the  north  drift  by  means  of  this  faulting.  Some 
perplexity  has  arisen  concerning  the  nature  and  relation  of  these  two  rocks. 
After  study,  however,  the  author  is  of  the  opinion  that  the  latter  rock  is  a  phase 
of  the  earlier  andesite,  while  the  white  rock  is  a  coarse-grained  phase  of  the  glassy 
Tonopah  rhyolite-dacite. 

Microscopic  examination  shows  that  this  hard,  white  rock  is  considerably 
altered.  The  phenocrysts  are  of  altered  feldspar,  in  part  andesine-oligoclase  and 
in  part  orthoelase;  the}'  arfe  now  largely  changed  to  muscovite  (sericite)  and 
adularia.  Small  original  biotite  crystals  are  thoroughly  bleached.  The  glassy 
groundmass  contains  veinlets  of  calcite  and  abundant  pyrite.  The  chemical 
analysis  of  the  rock,  by  Mr.  George  Steiger,  is  as  follows: 

Analysis  of  altered  Tonopah  rhyolite-dacite. 
[Specimen  2V9.J 

SiO, 68.19 

A1,O, 15.13 

FeA 1.31      . 

FeO 42 

MgO 29 

CaO 1.19 

Na/) 3.13 

K,O «.66 

TiO, 32 

PA 15 


VALLEY    VIEW    VEIN    SYSTEM.  149 

WANDERING    BOY    VEINS. 

About  200  feet  northeast  of  the  Wandering  Boy  shaft  there  outcrop  several 
quartz  veins  whose  position  and  course,  an  will  be  seen  on  the  map  (PI.  XI),  suggest 
that  they  form  a  continuation  of  the  Valley  View  system. 

RELATIVE    ELEVATION    OK    FAULT    BLOCKS   CONTAINING    VALLEY    VIEW  AND    WANDERING    BOY    VEINS. 

The  veins  above  mentioned  are  in  earlier  andesite,  probably  in  the  same  fault 
block  as  is  Gold  Hill.  That  Gold  Hill  is  bounded  on  the  north  by  a  fault  is  shown 
by  stratigraphic  evidence,  for  the  Siebert  tuff  on  the  north  is  in  rectilinear  contact 
with  the  earlier  andesite  on  the  south,  indicating  a  very  considerable  displacement. 
Along  this  fault  line  a  valley  has  been  eroded,  up  which  the  road  runs.  The  fault 
block  north  of  this  fault  is  bounded  on  the  west  by  the  Stone  Cabin  fault,  which  has 
an  upthrow  on  the  west,  bringing  up  the  earlier  andesite  of  Mizpah  Hill.  There- 
fore the  movement  of  the  Stone  Cabin  fault  compensates  to  a  large  degree  for 
that  of  the  Gold  Hill  fault;  and  a  prolongation  of  the  Gold.  Hill  fault  north- 
westward between  Mizpah  Hill  and  the  Wandering  Boy  finds  the  earlier  andesite 
corning  together  and  lying  on  both  sides  of  this  prolongation.  There  is,  however, 
some  reason  for  believing  that  the  fault  actually  continues  along  this  line,  though 
with  much  diminished  displacement. 

RELATION   OK   VALLEY   VIEW  AND   WANDERING   BOY   VEINS. 

According  to  the  tentative  conclusion  stated  in  the  last  sentence  above,  the  out- 
cropping veins  northeast  of  the  Wandering  Boy,  if  they  are  a  part  of  the  Valley 
View  system,  are  separated  by  a  west- north  west  fault  from  the  Valley  View  veins 
of  Mizpah  Hill.  They  are  represented  on  PI.  XVII  and  on  fig.  ±2  (p.  153).  The 
strike  is  northeast  and  the  dip,  like  that  of  the  Fraction  veins,  and  unlike  that  of 
most  of  the  veins  of  Mizpah  Hill,  is  to  the  south  at  angles  of  from  50C  to  75°.  In 
size  and  course  they  are  not  unlike  the  westernmost  outcrops  of  the  Valley  View 
veins  on  the  western  edge  of  Mizpah  Hill,  about  300  feet  away.  The  southerly  dip, 
also,  is  found  represented  in  this  portion  of  the  Valley  View  outcrops,  the  western- 
most veins  dipping,  at  the  surface,  south  at  angles  of  from  70°  to  80°. 

At  a  depth  of  a  few  hundred  feet  the  veins  which  occur  in  the  Wandering  Boy 
workings,  and  which  are  probably  identical  with  those  outcropping  northeast  of  the 
shaft,  acquire  a  flatter  dip— 30°  to  -tOc  to  the  south— and  thus  correspond  in  dip 
with  the  vein  shown  in  the  Fraction  workings.  On  Mizpah  Hill,  however,  the 
Valley  View  veins,  at  a  corresponding  distance  underground,  have  a  similar  dip  of 
about  30°  in  the  opposite  direction — to  the  north.  The  veins  in  the  two  localities 
can  not  be  directly  correlated,  and  their  prolongations  on  a  given  uniform  level 
underground  would  be  several  hundred  feet  apart,  though  nearly  parallel. 


150  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

Opposing  dips  of  the  veins  probably  original. — The  reason  for  the  opposing 
underground  dips  of  these  veins,  which  have  nearly  the  same  line  of  outcrop 
and  a  nearly  identical  dip  at  the  surface,  is  not  clear.  As  before  stated,  the 
Wandering  Boy  and  the  Valley  View  veins  seem  to  lie  in  different  fault  blocks, 
being  separated  by  a  probable  fault  which  runs  along  the  road  between  them; 
and  it  is  possible  that  the  faulting  may  have  been  of  such  a  differential  nature 
as  to  partially  revolve  the  block  containing  the  Wandering  Boy  veins  and  to 
reverse  the  dip.  Evidence  obtained  both  in  the  Wandering  Boy  and  in  the 
Fraction  demonstrates  that  the  dip  of  a  vein  may  be  changed  and  even  reversed 
by  faulting,  and  by  accompanying  deformation  which  corresponds  nearly  to 
folding,  but  which  is  probably  the  result  of  an  aggregate  of  small  faults. 

Against  this  interpretation  is  the  fact  that  the  steep  south  dip  of  the  Wander- 
ing Boy  veins  at  their  outcrop  corresponds  with  the  similar  surface  dips  of  the 
heavy  Valley  View  vein,  which  is  the  vein  of  the  outcropping  Valley  View  group 
lying  farthest  east,  and  the  one  with  which  the  Wandering  Boy  vein  would  naturally 
be  correlated.  If  the  different  dip  of  the  veins  in  the  two  blocks  is  due  to  the 
revolving  of  one  block  on  another  this  difference  should  appear  at  the  surface  as 
well  as  underground;  that  it  does  not  is  evidence  rather  in  favor  of  the  conclusion 
that  the  displacement  has  occurred  without  any  notable  change  in  the  attitude  of 
the  veins  aside  from  local  and  minor  effects.  In  this-  case  it  follows  that  the 
veins  of  the  Valley  View  system  present,  if  the  perplexing  faulting  were 
eliminated,  marked  differences  in  dip,  the  main  Wandering  Boy  vein  dipping  at 
a  moderate  angle  to  the  south,  as  the  main  Valley  View  vein  does  toward  the 
north. 

Change  of  dip  shown  by  the  comparison  of  the  Valley  View  and  the  Stone  Cabin. — 
In  this  connection  the  studies  already  made  on  the  Valley  View  veins  are  impor- 
tant. It  has  been  shown  that  the  outcropping  heavy  vertical  veins  of  this  system 
on  Mizpah  Hill  do  not  persist,  as  demonstrated  by  the  Valley  View  workings,  to 
a  depth  of  as  much  as  200  feet,  but  are  represented  at  this  depth  and  below  by  a 
strong  vein  dipping  about  35°  to  the  north.  In  the  Stone  Cabin  and  Silver  Top 
workings,  however,  a  vein,  which  is  certainly  the  continuation  of  the  outcrop- 
ping heavy  Valley  View  vein,  continues  down  almost  vertically  to  a  demonstrated 
depth  of  over  400  feet,  beyond  which  point  exploration  has  not  been  made.  This 
portion  of  the  vein  is  separated  from  the  larger  portion  in  the  Valley  View 
workings  by  a  fault,  along  which  the  displacement  of  the  vein  seems  to  have 
been  normal,  so  that  the  vertical  portion  shown  in  the  Stone  Cabin  workings 
has  been  dropped  down  below  the  north-dipping  portion  of  the  Valley  View. 

According  to  this  the  part  of  the  main  Valley  View  vein  which  has  been 
eroded  to  expose  the  present  outcrop  on  Mizpah  Hill  must  originally  have  extended 


WANDERING    BOY   VEINS. 


151 


vertically  up  above  the  present  surface  for  a  distance  of  several  hundred  feet,  at 
least. 

Wandering  Boy  and  Valley  View  conjugated  veins. — If  the  conditions  on  the 
west  side  of  Mizpah  Hill,  where  the  Valley  View  veins  approach  the  Wandering 
Boy  veins,  are  like  those  on  the  east  end  near  the  Stone  Cabin,  the  Wandering 
Boy  block,  if  depressed,  should  have  brought  down  the  vertical  portion  of  the 
vein,  a  condition  which  is  not  found.  What  the  relative  movement  of  the  two 
blocks  has  actually  been  is  not  certain.  Siebert  lake  beds  are  exposed  in  the 
southwest  corner  of  the  Mizpah  Hill  block,  and  are  assumed,  from  the  topog- 
raphy, to  occur  in  the  southeast  corner,  but  have  not  been  actually  observed 


FIG.  41.— Hypothetical  diagrammatic  vertical  cross  section  of  the  Valley  View  vein  system  (represented  by  its  principal 
and  strongest  vein)  before  faulting  and  erosion.  The  upper  part  is  considered  to  b«  now  represented  in  the  Stone 
Cabin  and  Silver  Top  workings  and  for  a  short  distance  below  the  outcrop  of  Mizpah  Hill.  The  north  vein  is  con- 
sidered to  be  represented  by  the  main  vein  in  the  Valley  View  workings,  the  south  vein  by  that  of  the  Wandering 
Boy  and  Fraction. 

there.  This  indicates  that  the  Mizpah  Hill  block  has  been  depressed,  relatively 
to  the  Gold  Hill  block,  so  that  the  Wandering  Boy  vein  would  represent  an 
originally  lower  portion  of  the  Valley  View  vein  system  than  the  portion  now 
outcropping  on  Mizpah  Hill.  If  this  is  so,  the  vertical  portion  of  the  Valley 
View  vein  system  should  be  expected  to  pass  in  depth  to  veins  dipping  south  at 
angles  of  30°  or  40°.  From  the  Valley  View  workings,  however,  it  is  known 
that  in  depth  the  vertical  veins  here  actually  pass  into  north-dipping  veins  and 
continue  so  several  hundred  feet  downward,  at  least.  The  north-dipping  and  the 


152  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,  NEVADA. 

south-dipping  flat  veins,  represented,  respectively,  in  the  Valley  View  and  in 
the  Wandering  Boy,  are  then  probably  not  parts  of  the  same  vein,  but  represent 
a  pair  of  veins  dipping  at  equal  angles  in  opposite  directions  (fig.  41). 


OUTCROPS    OF    WANDERING    BOY    VEINS. 


The  outcrop  veins  northeast  of  the  Wandering  Boy  all  have  a  northeast  strike 
and  a  southeast  dip.  As  observed  at  the  surface  they  are  designated  as  1,  2,  3, 
and  4  on  fig.  42. 

REPRESENTATION   OF   OUTCROPPING   VEINS   UNDERGROUND. 

The  heaviest  vein,  No.  1,  as  there  is  reason  to  believe,  may  be  the  main 
vein  of  the  underground  workings  shown  in  the  300-foot  level.  The  8-inch  vein 
represented  on  the  300-foot  level,  northwest  of  the  probable  position  of  the  main 
vein  at  this  point,  may  very  well  be  the  same  as  No.  2.  The  6-inch  vein  followed 
on  the  115-foot  level  may  perhaps  also  be  No.  2,  in  spite  of  the  fact  that  though 
it  has  a  southeast  dip  it  lies  almost  directly  over  the  supposed  No.  2  vein  on  the 
300-foot  level.  The  general  result  of  the  faulting  here  has  been  to  place  the  veins 
in  the  lower  levels  in  a  position  farther  north,  on  the  west  side  of  the  numerous 
faults,  than  would  be  the  case  if  the  veins  continued  regularly  downward.  The 
inclined  shaft  shown  in  the  figure  was  inaccessible  at  the  time  of  the  writer's 
visit,  but  drifts  were  run  on  two  veins  at  distances  of  65  and  95  feet  from  the 
surface.  It  is  likely,  as  shown  in  the  figure,  that  the  former  was  on  the  No.  3 
vein."  the  latter  on  the  No.  4. 


FAULT    SYSTEMS    IN    THE    WANDERING    BOY. 


In  the  Wandering  Boy  workings  the  veins  are  thrown  into  great  confusion 
by  faulting.  Analysis  of  the  disturbance  leads  to  the  conclusion  that  the  faulting 
can  be  referred  to  two  major  systems — that  of  the  Wandering  Boy  fault,  which 
strikes  northwest  and  outcrops  just  east  of  the  Wandering  Boy  shaft,  and  that 
of  the  Fraction  fault,  which  strikes  northeast  and  whose  outcrop  is  drawn  on 
the  map  as  lying  between  the  Fraction  No.  1  and  the  Fraction  No.  2  shafts. 
In  the  Wandering  Boy  workings  the  Wandering  Boy  fault  dips  southwest  at  an 
angle  of  approximately  50°,  while  in  the  Fraction  workings  the  Fraction  fault 
dips  southeast  at  an  angle  of  about  45°.  In  the  north  corner  of  the  block  inclosed 
by  these  two  faults,  therefore,  the  line  of  intersection  of  the  faults  pitches  south, 
and  the  faults  rapidly  approach  as  they  go  deeper.  The  estimated  position  of 
these  two  faults  on  the  300-foot  level  is  shown  in  PI.  XXI,  and  may  be  compared 
with  the  surface  outcrops,  as  shown  on  the  map  (PI.  XVI). 

a  Mr.  J.  M.  Healy  inform*  the  writer  that  the  vein  shown  in  the  figure,  as  drifted  on  at  the  66-foot  level,  was  3  feet 
thick  and  of  low  grade. 


WANDERING    BOY    VEINS. 

DISPLACEMENT    OF   THE    WANDERING    BOY    FAULT. 


153 


The  Wandering  Boy  fault,  as  .shown  on  the  map,  separates  the  earlier  andesite 
on   the   northeast  from    the    Fraction   dacite    breccia   on    the   southwest.     It    has 


Scale 

o      10      20  40  co  ao  loofeet 


FIG.  42.— Plan  showing  outcroppinK  veins  near  the  Wandering  Boy  and  their  probable  relation  to  the  veins  encountered 

underground. 

therefore  a  downthrow  on  the  .southwest,   and  underground  workings  show  that 
it  has  a  southwest  dip,  making  it  a  normal   fault  (fig.   43).     That  the  contact  is 


154 


GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 


^V-^J- ^.'fracture'  breccia 

''  v>-/-'^';  '•:*,-.  I  I  "./  \/.       —  r~.  / 


^i\  '^'-C'.K  •<•?<•/.•  W-v/. 
'Sv-'aoo-ff levei 


pii^lfsp 

^^Wl^'?^ 

:5^P§i;|^ 

Earlier  :-;andesiteA  /•' 


sw 


reallj-  due  to  a  fault  of  very  great  displacement  is  shown  by  the  occurrence 
underground  along  it  of  a  thick  friction  breccia  containing  fragments  of  later 
andesite,  of  granitic  rock,  and  of  the  adjacent  rocks. 

On  the  115-foot  level  the  main  Wandering  Boy  fault  is  well  developed  (fig.  44). 
The  small  6-inch  vein  followed  on  this  level  shows  a  repeated  breakdown  to  the 
southwest  a.s  the  main  fault  is  approached,  a  movement  corresponding  to  the  chief 
normal  faulting.  Besides  this  there  are  horizontal  grooves  along  the  main  fault 
plane,  and  similar  striations  are  found  on  it  where  it  is  cut  in  the  shaft  below  at 
a  depth  of  185  feet.  Furthermore,  on  the  115-foot  level,  the  vein  is  bent  and 

dragged  to  the  northwest  along  the  fault  plane 
(fig.44),  and  here  the  dip  becomes  north  instead  of 
south,  as  normal.  These  phenomena  show  a  hori- 
zontal movement  to  the  northwest  on  the  southwest 
side  of  the  fault,  and  the  reversal  of  the  dip  shows 
some  differential  or  torsional  movement.  The  striae 
on  a  fault  plane  indicate  the  last  movement,  the 
records  of  previous  and  often  more  important  move- 
ments being  erased  by  each  new  one.  The  combined 
result  of  all  the  movements  indicated,  therefore,  is 
that  the  block  on  the  southwest  side  of  the  fault  has 
moved  downward,  and  also  to  a  less  degree  (probably) 
northwestward,  along  the  fault  plane.  This  hori- 
zontal movement  is  also  shown  in  the  Fraction,  where 
the  northwest  faults  (see  p.  144)  are  probably  auxiliary 
slips  related  to  the  Wandering  Boy  system.  In  the 
Fraction,  especially  on  the  300-foot  level,  important 
horizontal  movement  is  registered  by  the  striation. 


100 


so 


Scale 
o 


CROSS   FAULTING   ON   THE  300-FOOT   LEVEL. 


100  feet 


Fiu.  «.— Vertical  section  on  the  Wan 
defing  Boy  shaft,  showing  the  main 
Wandering  Boy  fault. 


Complicated  faulting  is  shown  in  the  300-foot  level 
of  the  Wandering  Boy.  The  main  workings  consist  of 
two  drifts  run  at  right  angles,  one  running  nearly  east 
and  the  other  south.  The  vein  shown  in  this  level  has  a  thickness  of  3  or  4  feet, 
strikes  northeast  or  east-northeast,  and  dips  southeast  at  an  angle  of  30°  or  40°. 
The  east  drift,  therefore,  runs  somewhat  diagonally  to  the  strike  of  the  vein, 
though  more  nearly  along  it,  while  the  south  drift  also  runs  diagonally  though 
somewhat  more  across  the  strike  (fig.  48).  The  vertical  section  along  the  east 
drift  is  given  in  fig.  45,  that  along  the  south  drift  in  fig.  46.  Near  the  end  of 
the  south  drift  a  short  east  drift  has  been  run,  following  a  portion  of  the  vein, 
and  the  vertical  section  along  this  drift  is  given  in  fig.  47.  In  fig.  45  it  is  shown 


WANDERING    BOY    CROSS    FAULTS. 


155 


that  the  vein  (which  normally,  following  its  strike  and  dip,  would  disappear  from 
the  drift)  is  continually  thrust  up  to  the  east  by  close-set  faults,  so  as  to  persist 
in  the  drift. 


Workings. 


25 


100  feet 


FIG.  44.— Horizontal  plan  of  115-foot  level,  Wandering  Boy  workings,  showing  minor  vein  and  Wandering  Boy  fault. 

Judging  from  the  section  (tig.  45),  most  of  the  faults  are  apparently  reversed 
faults,  while  some  are  normal.     In  the  south  drift,  the  vein  has  been  repeatedly  thrown 


FIG.  45.— Vertical  section  along  east  drift,  300-foot  level,  Wandering  Boy  mine,  showing  faulting  of  vein. 

up  to  the  south  by  close-set,  normal  faults,  as  shown  in  tig.  46.     There  is  no  question 

as  to  the  identity  of  the  fragments  of  vein  in  the  east  drift  and  those  in  the  northern 

half  of  the  south  drift,  for  the    connection    is    nearly   continuous. 

The  fragment  shown  in  the  south  end  of  the  south  drift  is  30  or  40 

feet  distant  from  the  fragments  farther  north,  and  may  represent  a        || 

closely   parallel   vein;    on  the   other   hand,   it  is   identical  with   the 


rt-»-n 

It  I  fa 


FIG.  46.— Vertical  section  along  south  drift,  300-foot  level,  Wandering  Boy  mine, 
showing  faulting  of  vein. 


ion. 


FIG.  47. —  Vertical 
section  showing 
short  crosscut  to 
east  near  south 
end  of  south 
drift.  300 -foot 
level,  Wander- 
ing Boy,  showing 
faulting  of  vein. 


other  vein  blocks  in  size,  strike,  dip,  and  appearance,  and  there  is  no  necessary 
reason  for  separating  it  from  them.  The  short  drift  running  east  on  this  southern- 
most vein  fragment  shows  conditions  identical  with  those  in  the  main  east  drift 
(fig.  47),  the  vein  being  upfaulted  to  the  east  by  close-set,  apparentlv  reversed 
faults. 


156 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 


The  situation  is  shown  in  horizontal  plan  in  fig.  -48,  where  the  strikes  of  the 
faults  and  the  vein  blocks  may  be  studied  and  compared,  as  the  dips  may  be 
compared  in  the  vertical  sections.  Here  it  is  seen  that  the  faults  on  the  east  drifts 
have  essentially  a  north-south  course,  some  trending  to  the  west  of  north,  and 
perhaps  most  of  them  to  the  east  of  north:  and  that  those  in  the  south  drift  are 


T:      •    i  \ 
~~^=^^70' 


Scale 


0  10          20 


•o  feet 


Flu.  48.— Horizontal  plan  of  WanderiiiK  Hoy,  300-foot  level,  showing  fragmentu  of  vein  mill  <-ross  limits,  witli  tin- 
general  trend  of  equal  displacement. 

essentially  east  and  west  faults,  though  usually  trending  north  of  west.  Therefore 
the  vein  may  be  considered,  for  the  sake  of  clearness,  as  cut  by  two  intersecting 
systems  of  faults,  one  striking  north  and  south  and  the  other  east  and  west.  The 
vein  is  repeatedly  upthrown  on  the  east  by  the  north-south  system  and  on  th» 
south  by  the  east-west  system. 


EFFECTS    OF    CROSS    FAULTING. 


157 


Effect*  »f  CTOSS  faulting  ideally  considered. — In  order  to  understand  the 
resultant  effect  of  such  intersecting  faults,  let  us  take  a  simplified  example  such 
as  is  shown  in  the  stereogram,  fig.  49.  This  shows  a  rectangular  block  which 
has  been  affected  by  two  sets  of  vertical  faults,  striking  at  right  angles  to  each 
other.  On  the  figure  they  are  also  represented  as  equally  spaced  and  all  having 
the  same  displacement,  thus  giving  to  the  example  an  ideal  simplicity  which  is 


FIG.  49.— Stereogram  showing  the  results  of  cross  faults  equally  spaced  and  of  equal  throw. 

probably  rarely  found  in  nature.  The  result  of  these  intersecting  faults,  as  is 
seen,  is  that  lines  or  planes  of  equal  displacement  are  zigzag,  being  made  up  of 
regularly  alternating  portions  of  each  of  the  two  fault-system  planes,  the  length 
of  each  of  the  component  straight  lines  being  determined  by  the  spacing  of  the 
faults,  while  the  trend  of  the  whole  zigzag,  and  therefore  of  the  lines  of  equally 
displaced  blocks,  is  diagonal  to  both  the  fault  systems.  In  effect,  the  resultant 


158 


GEOLOGY   OF   TONOPAH   MINING   DISTRICT,   NEVADA. 


of  these  two  intersecting  directions  of  faulting  has  been  a  third  diagonal  system, 
which  represents  the  direction  of  equal  faulting. 

On  a  plane  projection,  with  closer  spacing  of  faults,  or  a  greater  number  of 
thorn  shown  on  a  smaller  scale,  the  situation  is  again  shown  in  fig.  50.  It  may 
be  remarked  that  to  obtain  such  a  resultant  there  need  not  necessarily  be  any 

N 

Downthrow 


W 


O 
o 

IE 


FIG.  50. — Diagram  showing  horizontal  plan  of  equal  and  equally  spaced  faults  belonging  to  two  systems  intersecting  at 
right  angles,  the  north-south  system  having  a  regular  downthrow  on  the  east,  and  the  east-west  system  a  regular 
downthrow  on  the  south  side.  The  heavy  zigzag  line  represents  one  of  the  lines  of  equal  faulting,  the  shaded  squares 
one  of  the  zones  or  blocks  of  equal  displacement. 

correspondence  between  the  direction  of  displacement  (whether  up  or  down)  of 
the  two  systems.  If  we  reverse  the  movement  of  either  of  the  fault  systems, 
for  example,  if  in  the  figure  (to  be  understood  with  the  aid  of  the  stereogram) 
the  north-south  faults  are  downthrown  to  the  west  instead  of  to  the  east,  a 


EFFJ5OI8    OF   CROSS   FAULTING. 


159 


similar  resultant  faulting  will  be  accomplished,  but  with  a   trend  at   right  angles 
to  that  depicted. 

Downthrow 


O 
o 

H- 

IT 
O 


Line  of  equal 
displacement 


FIG.  51. — Diagram  showing  course  of  line  of  equal  faulting  for  two  systems  of  faults  intersecting  at  right  angles  and 
having  uniform  displacements,  the  spacing  being  uniform  within  each  system  but  different  for  each  system. 

11 


•«FE 

%s 


Downthrow 


Line  of  equal 
displacement 


Line  of  equal 
displacement 


FIG.  52.— Diagram  showing  the  diverse  courses  of  lines  of  equal  displacement  which  are  the  result  of  two  systems  of  equal 
.faults  intersecting  at  right  angles  but  unequally  spaced. 

From  this  simple  case  the  variations  and  irregularities  such  as  are  usually  met 
in  nature  bring  about  endless  changes.  A  few  of  these  majr  be  ideally  deduced. 
Fig.  51  illustrates  a  case  of  equal  faulting  in  two  systems  which  are  at  right  angles, 


160 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 


the  spacing  of  the  faults  within  each  system  being  equal,  but  that  of  one  system 
being  different  from  the  other.     Fig.  52  represents  a  case  similar  in  all  respects,  save 


\ 


Downthrow 


Downthrow 


Line  of  equal 
displacement 


FIG.  53. — Diagram  showing  the  line  of  equal  displacement  when  the  fault  systems  are  oblique  to  each  other  instead  of 
being  at  right  angles,  the  conditions  otherwise  being  like  those  in  fig.  50. 

that  the  spacing  of  the  faults  of  both  systems  is  irregular.     Fig.  53  shows  a  case 
similar  to  rig.  50.  save  that  the  fault  systems  are  oblique  instead  of  perpendicular. 

If.  now.  the  amount  of  displacement  in  the  two  fault  systems  is  different,  even 
though  it  be  constant   within  each  system,   blocks  of   equal  displacement  will  no 

•  N 

Downthrow 


Kui.  .14.— Diagram  showing  the  effect  of  cross-faults  when  the  faults  of  one  system  have  twice  the  displacement  of  those 
nl  the  other  system.  Here  the  north-south  faults  have  double  the  displacements  of  the  east-west  ones.  The  shaded 
blocks  are  blocks  of  equal  displacement. 

longer    be   connected,    and    therefore   there   will    be    no   continuous   line   of   equal 
displacement.     In  rig.  54,  for  example,  where  the  displacement  of  the  faults  is  twice 


EFFECTS    OF    CROSS    FAULTING. 


161 


as  great  in  one  system  as  in  the  other,  the  isolated  blocks  of  equal  displacement  will 
be  separated  from  one  another,  as  are  the  starting  and  stopping  squares  of  the 
knight  move  on  a  chess  board.  If  the  displacement  of  one  system  is  three  times, 
instead  of  twice,  as  great  as  the  other,  the  blocks  of  equal  displacement  will 
be  removed  (in  the  diagram)  one  square  farther  from  one  another,  in  a  direction 
parallel  to  the  faults  of  greater  displacement,  and  so  on.  If,  again,  the  faults  in 
each  system  are  unequal  amotuj  themselves  in  regard  to  their  amount  of  displacement, 
the  fault  blocks  bounded  by  the  two  systems  will  be  distributed  in  many  apparently 
irregular  ways,  and  each  block  will  appear  as  a  separate  unit  that  has  moved 
independently,  rather  than  as  the  resultant  of  intersecting  faults.  Still,  in  all  cases, 
it  appears  to  hold  good  that  in  general  the  zones  of  blocks  of  equal  displacement, 
roughly  aligned  though  these  ma\'  be,  will  lie  diagonally  between  the  two  fault 
systems.  Which  diagonal  it  will  be  can  be  ascertained  from  the  following  diagram, 
fig.  55: 


Fra.  55.— Diagram  showing  trend  of  zones  of  equal  displacement  with  given  directions  of  downthrow. 

As  illustrated  in  fig.  53,  these  conclusions  hold  good  for  faults  striking  obliquely 
to  one  another  as  well  as  at  right  angles.  They  also  hold  good  for  faults  which  dip 
obliquely  instead  of  perpendicularly,  and  for  cases  where  the  dips  in  the  two  sets  are 
different  in  angle  or  direction,  or  both. 

Application  of  principles  to  Wandering  Boy  cross  faults. — These  deduced 
general  principles  enable  us  to  understand  the  result  of  the  intersecting  faults  in 
the  Wandering  Boy  300-foot  level.  The  resultant  of  the  east-west  and  the  north- 
south  faulting  is  a  northeast  trend  of  equal  displacement,  as  indicated  on  the  figure, 
16843— No.  42—05 1 1 


162  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

nearly  parallel,  as  it  happens,  with  the  strike  of  the  vein.  Blocks  lying  in  zones 
with  this  general  trend  have  been  systematically  elevated  above  adjacent  parallel 
zones  lying  to  the  northwest. 

The  vein,  dip  as  a  factor  in  th« problem. — In  the  case  of  the  faulting  on  the 
Wandering  Boy  300-foot  level,  the  problem  takes  on  an  added  complexity,  since  the 
available  test  of  faulting  is  not  the  relative  position  of  the  displaced  blocks,  but  rather 
the  position  of  the  vein,  whose  plane  is  oblique  to  any  of  the  planes  of  the  fault  blocks, 
and  whose  present  position  is  what  we  seek  ultimately  to  understand.  The  strike  of 
the  vein  being  nearly  parallel  with  the  trend  of  equal  displacement,  it  results  that  if 
the  dip  is  toward  the  direction  of  resultant  equal  downthrow,  then  the  two  factors  of 
lowering  the  vein  will  be  added  and  the  fragments  of  the  vein  will  gain  depth  faster 
than  the  inclosing  rock  blocks.  If,  on  the  other  hand,  the  dip  is  against  the  down- 
throw, two  factors  of  lowering  the  vein  will  be  set  off  against  each  other.  The  vein 
then  will  gain  depth  more  slowly  than  the  inclosing  rock  blocks,  if  the  faulting  has 
a  greater  effect  than  the  dip;  will  continue  on  a  general  horizontal  plane,  if  the 
faulting  has  an  effect  about  equivalent  to  that  of  the  dip;  or  will  ascend,  in  spite  of 
the  downfaulting,  if  the  latter  be  sufficiently  slight  to  have  its  effect  overbalanced 
by  the  dip.  In  the  Wandering  Boy  300-foot  level,  we  have,  as  may  be  seen  from 
the  sections,  the  second  of  these  conditions.  The  dip  is  opposite  to  the  downthrow, 
and  the  angle  of  dip,  the  displacement,  and  the  spacing  of  the  faults  are  fortuitously 
such  (for  a  distance  at  least)  that  the  one  offsets  the  other,  and  the  vein  continues  in 
a  horizontal  zone.  This  explains  why  the  long  east  and  south  drifts  and  the  short 
east  crosscut  from  the  south  drift  all  encounter  blocks  of  apparently  the  same  vein; 
and  it  follows  that  other  blocks  of  the  vein  probably  exist  on  this  same  level  in 
the  angle  between  the  two  main  drifts,  and  beyond  the  explored  area  as  far  as  this 
peculiar  intersecting  faulting  and  the  balance  of  dip  and  displacement  is  maintained. 

CORRELATION    OK   VEINS   IN   FRACTION    AND   IN    WANDERING   BOY. 

PI.  XXI,  p.  140,  shows  the  vein  and  faults  of  the  corresponding  300-foot  levels  of 
the  Fraction  and  the  Wandering  Boy,  together  with  the  estimated  position  of  the  lines 
of  main  faulting  of  both  the  Wandering  Boy  and  the  Fraction  faults.  It  is  here  seen 
that  the  northeast  faults  in  the  Wandering  Boy,  which  form  the  majority  of  those 
faults  classed  together,  in  describing  the  cross  faulting  on  the  300-foot  level,  as  north- 
south  faults,  are  parallel  in  strike,  dip,  and  direction  of  displacement,  with  the  chief 
set  of  faults  in  the  Fraction,  and  in  strike  at  least  with  the  main  Fraction  fault  as 
determined  on  the  surface.  These  minor  faults  involve  a  movement,  as  seen  on  a 
horizontal  plane,  to  the  north  on  the  west  side;  as  seen  on  a  vertical  section,  down- 
ward on  the  west  side.  The  real  movement  has  probably  been  a  compound  of  these 
two,  as  studied  out  in  the  Fraction  workings.  Along  the  main  fault  plane,  then  (if, 
indeed,  there  is  one,  and  the  displacement  is  not  rather  distributed  over  many  par- 


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WANDERING    BOY    FAULTS.  163 

allel  faults),  the  movement  was  undoubtedly  similar  to  that  of  the  minor  faults,  and 
would  bring  the  two  portions  of  a  faulted  vein  into  somewhat  the  position  that  the 
Wandering  Boy  and  the  Fraction  veins,  taken  as  a  whole,  occupy  to  each  other.  This 
leads  to  the  suspicion  that  the  two  occurrences  were  originally  the  same  vein  and 
were  separated  by  the  Fraction  fault.  The  veins  in  the  two  mines  are  similar  in 
strike,  dip,  size,  and  general  characteristics.  A  fragment  of  the  Fraction  vein  lying 
farthest  south  on  the  237-foot  level  and  probably  in  a  zone  east  of  any  exploration 
on  the  300-foot  level  has  been  plotted  on  the  map.  There  is  also  shown  its  approx- 
imate position  on  the  300-foot  level,  if  it  continues  downward  that  far  with  the 
observed  dip.  This  fragment  lies  midway  between  the  main  portions  developed  in 
the  two  mines,  supporting  the  theory  of  the  original  identity  of  the  veins. 


FAULTS    NOT   CORRESPONDING   TO   THE    MAIN    SYSTEMS. 


The  northwesterly  faults  of  the  Wandering  Boy  300-foot  level  are  not  so 
closely  related  to  the  Wandering  Boy  fault  as  the  northeasterly  faults  are  to  the 
Fraction  fault.  Their  trend  is  various,  sometimes  coinciding  with  that  of  the  main 
Wandering  Boy  fault,  sometimes  not.  Their  dip,  as  shown  in  fig.  48,  is  usually 
steeply  northeast,  or  in  the  opposite  direction  from  that  of  the  main  fault,  so  that 
while,  like  the  main  fault,  they  are  normal,  the  downthrow  is  on  the  northeast 
instead  of  on  the  southwest  in  accordance  with  the  larger  movement.  Many  of 
them  are,  therefore,  perhaps  to  be  accurately  regarded  as  independent  minor  faults., 
resulting  from  the  combined  stresses  of  the  major  displacements. 

RELATIVE    AGE   OF    FRACTION    AND    WANDERING    BOY    FAULTS. 

The  Fraction  fault  movement  partakes  essentially  of  the  nature  of  thrust 
faulting,  and,  as  has  been  explained,  seems  to  be  due  to  the  horizontal  shove  exerted 
by  the  intrusion  of  the  Brougher  Mountain  volcanic  neck.  The  Wandering  Boy 
fault,  on  the  other  hand,  is  a  normal  fault,  such  as  is  ordinarily  due  to  gravity;  and 
the  fact  that  faulted  blocks  are  downthrown  on  the  south,  in  the  direction  of  the 
dacite  volcanic  centers,  leads  to  the  belief  that  the  downthrow  was  a  part  of  the 
general  downfaulting  in  the  neighborhood  of  these  volcanoes,  which,  as  described 
on  page  47,  probably  took  place  subsequent  to  the  last  important  outbursts  as  a 
result  of  collapse  due  to  the  expulsion  of  a  large  bulk  of  material  from  the  under- 
lying region.  According  to  this  the  Wandering  Boy  fault  is  slightly  but  distinctly 
subsequent  to  the  Fraction  fault. 

ORE   IN   WANDERING   BOY   VEINS. 

Like  most  of  the  Fraction  vein  material,  and  much  of  the  material  in  the  Valley 
View,  most  of  the  quartz  in  the  Wandering  Boy  thus  far  developed  is  low  grade,  or 
even  practically  barren.  Good  assays  are  obtainable,  but  even  limited  masses  of 


164  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,   NEVADA. 

rich  ore,  like  those  which  occurred  in  the  Fraction,  were  not  encountered  to  any 
extent.  Metallic  minerals,  other  than  a  limited  amount  of  iron,  are  not  often  noted 
in  the  veins.  Some  ruby  silver  and  argentite,  like  that  in  the  Fraction,  have  been 
reported,  but  were  not  seen  by  the  writer. 


VEINS   OF  GOLD   HILL. 
GOLD   HILL    A    FAULT    BLOCK. 

The  Gold  Hill  block  is  of  especial  interest,  as  being  the  only  outcropping 
block  of  earlier  andesite  besides  the  Mizpah  Hill  block.  It  is,  roughly  speaking, 
a  triangular  area.  It  is  bounded  on  the  north  and  south  by  faults  and  on  the 
east  by  the  intrusive  dacite  of  Golden  Mountain.  The  fact  that  the  contact  of 
this  dacite,  as  shown  on  the  map,  is  nearly  a  straight  line,  suggests  strongly  the 
idea  that  it  has  been  determined  by  a  preexisting  fault.  This  idea  is  strengthened 
by  an  inspection  of  the  boundary  just  northeast  of  the  Tonopah  and  California 
shaft,  where  the  intrusive  dacite  contracts  to  a  narrow  dike,  which  separates  the 
block  in  which  the  Tonopah  and  California  shaft  is  situated  from  the  Gold  Hill 
block.  The  former  block  has  at  its  surface  the  white  tuffs  (Siebert  tuffs)  of  the 
lake  beds,  under  which  the  Tonopah  and  California  encountered  the  earlier 
andesite.  This  block  is  therefore  depressed  with  reference  to  the  Gold  Hill  block, 
and  the  dacite  dike  has  been  intruded  along  the  fault  plane. 

NATURE   OF   GOLD   HILL   ANDKSITE. 

The  character  of  the  andesite  of  Gold  Hill  has  been  the  subject  of  critical 
study.  On  the  western  extremity  of  the  block  at  a  point  south  of  Mizpah  Hill, 
the  andesite  has  the  same  peculiar  appearance  as  at  Mizpah  Hill.  Farther  east, 
toward  the  top  of  the  hill,  the  andesite  takes  on  a  different  appearance,  being 
darker  and  showing  somewhat  larger  feldspar  phenocrysts  and  frequent  pheno- 
crysts  of  altered  but  easily  recognizable  biotite.  The  latter  kind  of  andesite 
resembles  in  some  ways  the  later  andesite,  and  at  one  time  aroused  in  the  mind 
of  the  writer  the  same  doubt  as  to  its  affiliation  that  the  andcsites  of  the  Fraction, 
West  End,  and  MacNamara  did.  Critical  study,  however,  established  the  following 
points:  That  there  is  no  real  boundary  between  the  typical  Mizpah  Hill  variety 
of  andesite  and  the  biotite-bearing  andesite  of  the  eastern  part  of  the  Gold  Hill 
block;  that  under  the  microscope  the  last-named  phase  showed  many  other  char- 
acteristics of  the  earlier  andesite,  while  it  was  seen  to  contain,  as  ferromagnesian 
phenocrysts,  biotite  to  the  practical  exclusion  of  hornblende  or  pyroxene;  and  that 
the  Gold  Hill  andesite  contained  small  but  typical  quartz  veins  like  those  of  Mizpah 
Hill.  One  of  these  Gold  Hill  veins  has  produced  rich  ore,  although  in  limited 
quantity.  Moreover,  while  the  Gold  Hill  shaft  shows  in  its  upper  portion  the 


VEINS    OF    GOLD    HILL.  165 

peculiar  characteristics  of  .  the  .surface  andesite,  in  its  lower  portion  it  gradually 
passes  into  fresher  andesite,  more  like  that  of  Mizpah  Hill,  and  there  is  little 
question  that  the  two  phases  form  parts  of  the  same  body.  Therefore,  it  has  been 
concluded  that  there  is  here  a  phase  of  the  earlier  andesite  which  contains  biotite 
rather  than  hornblende,  and  which  also  has  a  somewhat  coarser  feldspar  crystal- 
lization. Similar  phases  can  be  found  on  Mizpah  Hill,  and  even  very  close  to 
the  Mizpah  vein,  and,  as  stated  elsewhere,  the  rock  can  be  matched  in  the  Fraction 
and  neighboring  shafts. 

ALTERATION    OF   GOLD   HILL   ANDESITE. 

The  alteration  of  the  Gold  Hill  andesite,  as  observed  in  surface  specimens, 
results  in  the  formation  of  quartz,  sericite  and  secondary  orthoclase  or  adularia. 
The  plagioclase  feldspars  (oligoclase-albite)  alter  to  orthoclase  (adularia)  and 
sericite,  or  to  sericite  and  quartz.  The  biotite  is  usually  altered  to  muscovite . 
and  quartz.  Occasional  pseudomorphs  of  secondary  minerals  after  hornblende 
were  detected,  consisting  chiefly  of  iron  minerals  (hematite,  etc.).  Numerous  small 
crystals  of  apatite  occur.  Practically  the  same  characteristics  are  found  in  the 
specimens  from  the  Gold  Hill  shaft,  with  rather  more  pseudomorphs  after  horn- 
blende and  some  chlorite  as  secondary  mineral. 

ENUMERATION   OF   THE   GOLD   HILL   VEINS. 

Gold  Hill  differs  in  an  important  manner  economically  from  the  Mizpah  Hill 
block,  in  its  comparative  poverty  in  mineralization.  The  veins  are  shown  on 
the  map,  but  are  narrow  and  weak.  The  most  important  outcropping  vein  may 
be  called  the  Good  Enough  vein,  from  the  name  of  one  of  the  claims.  At  one 
point  in  the  upper  part  of  the  Good  Enough  .shaft  the  vein  has  a  thickness  of 
H  feet,  but  diminishes  farther  down,  and  also  laterally  along  the  outcrop  in  both 
directions,  until  it  splits  into  diverging  and  unimportant  stringers.  This  vein  has 
an  east-northeast  strike  with  a  northerly  dip  at  an  angle  of  about  70°.  There 
is  a  parallel  vein  250  or  300  feet  to  the  northwest,  which  dips  in  the  opposite 
direction,  or  to  the  southeast,  at  an  angle  of  70°  or  80°.  This  vein  has  a  thickness 
of  3  to  6  inches  and  is  traceable  across  the  hill.  A  number  of  other  veins  of  the 
same  character  are  found,  one  of  which  runs  southeastward  from  the  Gold  Hill 
shaft,  parallel  to  and  just  above  the  road.  It  strikes  N.  60°  W.,  has  an  average 
thickness  of  <>  inches,  and  is  also  evidently  a  weak  vein.  Veins  of  the  same 
character,  nearly  parallel  to  that  last  mentioned,  occur  on  the  .southwest  side  of 
the  road,  in  the  same  block,  as  shown  on  the  map  (PI.  XVI).  They  are  usually 
several  inches  thick,  but  have  not  been  traced  far. 


166 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 


PRODUCTION  OF  GOOD  ENOUGH  VEIN. 

The  only  place  on  the  hill  from  which  much  ore  has  been  obtained  was  from 
one  section  of  the  surface  portion  of  the  Good  Enough  vein.  According  to  the 
Annual  Report  of  the  Director  of  the  Mint  on  the  Production  of  Precious  Metals 
for  1901,  the  vein  had  produced  and  shipped  $15,000  worth  of  ore  up  to  the 

time  of  the  publication  of  that  re- 
port. Not  much  further  work  has 
been  done  on  this  ore  body. 

VEIN    STRUCTURE. 

The  condition  of  the  Good  Enough 
vein  as  seen  in  the  chief  working 
shaft  is  shown  in  fig.  56.  From  the 
standpoint  of  origin  this  vein  is  in- 
teresting, as  it  shows  plainly  the 
effect  of  fracture  planes,  in  determin- 
ing not  only  the  walls,  but  in  pro- 
ducing a  diminution  in  the  size  of 
the  vein  and  even  a  change  of  course. 

There  is  no  faulting  in  the  section 
shown  in  the  figure,  and  the  change 
in  size  and  dip  of  the  vein  is  due 
simply  to  the  control  of  the  original 
mineralizing  circulation  first  by  one 
and  then  by  another  set  of  fractures 
(fig.  57).  This  is  in  accordance  with 
the  observations  made  on  the  Miz- 
pah  Hill  veins. 

GOLD   HILL   SHAFT. 

The  Gold  Hill  shaft  at  the  time 
of  the  writer's  visit  was  490  feet 
deep,  in  earlier  andesite  of  an  unusu- 

fractures.  ally  fresh  character  for  this  district. 

The  workings  consisted  of  crosscuts  to  the  north  and  south  at  this  level,  of  30 
feet  each.  There  was  another  level  at  a  depth  of  300  feet,  and  a  drift  '20  feet 
to  the  north  and  50  feet  to  the  south.  The  north  drift  at  this  level  showed  a 
2-inch  vein,  running  N.  80°  W.  and  dipping  north  at  an  angle  of  67°. 


Scale 
10  20 


30  feet 


FIG.  56.— Section  of  Good  Enough  shaft.  Gold  Hill.  Lower  out- 
lines of  shaft  indicated  by  dotted  lines.  Shows  eross  section 
of  vein  in  early  andesite,  with  minor  cross  walls.  Also 
shows  the  control  of  the  size  and  direction  of  the  vein  by 
dominating  fractures,  straight  lines  represent  some  of  the 


Surface 


VEINS    IN    THE    EARLIER    ANDESITE.  167 

TONOPAH  AND  CALIFORNIA  WORKINGS. 

SECTION    EXPOSED   IN   WORKINGS. 

The  Tonopah  and  California  shaft  is  situated  several  hundred  feet  southeast  of 
the  Gold  Hill  shaft.  It  starts  in  the  white  stratified  tuffs  of  the  lake  beds,  which 
here  have  a  north-northeast  strike  and  a  westerly  dip  of  about  20°.  According  to 
the  report  of  the  manager.  63  feet  of  these  tuffs  was  passed  through,  and  directly 
beneath  them  was  the  earlier  andesite.  A  short 
distance  south  of  the  shaft  the  tuffs  are  thicker,  as 
a  shaft  has  gone  down  100  feet  in  them  and  has 
not  reached  their  lower  limit. 

Some  quartz  stringers  were  found  in  the  ear- 
lier andesite  beneath  the  tuffs,  at  the  depth  of  about 
123  feet.  At  a  depth  of  about  135  feet  the  shaft 
enters  a  brecciated  zone,  which  consists  of  softened 
and  broken  earlier  andesite  and  occasional  bunches 
of  broken  quartz.  This  continues  down  in  the 
shaft  for  about  -40  feet.  At  a  depth  of  150  feet  a 
short  drift  runs  southward  in  this  broken  zone. 
The  minor  slips  within  this  zone,  have  a  north-south 
strike  and  a  dip  of  30°  to  the  east,  and  the  bottom 
of  the  zone  has  a  similar  strike  and  dip.  Below 
this  there  is  hard  earlier  andesite.  rather  dark  col- 
ored, with  occasional  north-south  slips  and  some 
broken  quartz  stringers,  evidently  faulted.  At  a 
depth  of  450  feet  a  drift  runs  in  a  southeasterly  direction  for  over  220  feet.  There 
is  another  level  at  a  depth  of  H50  feet. 

CALIFORNIA    FAULT. 

The  broken  zone  described  in  the  shaft  is  evidently  a  fault  zone.  Projected 
on  the  same  dip  to  the  surface,  this  zone  coincides  with  the  outcrop  of  the  fault 
which  separates  Gold  Hill  from  the  block  in  which  lies  the  top  of  the  Tonopah 
and  California  shaft.  At  the  surface,  however,  this  fault  zone  is  occupied  by  a 
dike  of  the  Golden  Mountain  dacite,  which  is  not  present  in  the  shaft.  Evidently 
the  dacite  is  straighter  than  the  fault  or  happens  to  be  missing  at  this  point. 

According  to  this  the  shaft  below  the  fault,  that  is  to  say,  below  180  feet,  is 
in  the  Gold  Hill  block.  Moreover,  the  east  drift  on  the  450-foot  level  does  not 
run  far  enough  to  cut  the  fault,  so  that  these  workings  are  in  the  same  block. 


FIG.  57. — Cross  section  of  Good  Enough  vein. 
Gold  Hill,  as  exposed  in  opening  just  west 
of  shaft,  showing  same  characteristics  as  in 
fig.  56.  Vertical  lines  in  andesite  are  joints. 


1H8  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

VEINS. 

Some  of  the  broken  fragments  in  the  fault  zone  show  a  small  quantity  of 
material  that  is  probably  black  silver  .sulphide. 

On  the  450-foot  level  a  small  quartz  vein,  a  few  inches  thick,  with  an  east- 
southeast  strike,  and  a  northerly  dip  (45°  to  60°),  was  followed.  This  has  a  gangue 
of  quartz,  with  some  calcite,  and  contains  pyrite.  In  some  places  good  values 
are  shown.  On  the  650-foot  level,  a  short  distance  south  of  the  shaft,  in  very 
dense,  and  tine-grained  earlier1  andesite,  a  ledge  of  3  feet  of  mixed  quartz  and 
altered  andesite  has  been  cut.  This  quartz  contains  argentite  and  shows  some 
good  values. 

MONTANA  TONOPAH  VEIN   SYSTEM. 
MONTANA  TONAPAH    MINE.    ' 

ABSENCE   OF   VEINN    IX    THE    LATER    ANDESITE. 

The  Montana  Tonopah  shaft  was  sunk  in  the  later  andesite,  on  the  northeast 
or  upper  side  of  the  Mizpah  fault  (PL  XVI).  It  passed  through  372  feet  of  the 
later  andesite  before  reaching  the  fault.  Most  of  this  rock  was  extraordinarily 
decomposed  and  thoroughly  bleached,  while  much  was  intensely  brecciated,  con- 
taining hard  bowlders  in  a  clayey  matrix,  with  strong  fractures  and  slickensided 
surfaces.  This  indicates  a  great  deal  of  faulting,  of  which  no  measure  could  be 
obtained. 

Above  the  Mizpah  fault  only  small  veinlets  of  calcite  and  quartz  were  encoun- 
tered, but  4  feet  below  the  fault  a  heavy  quartz  vein  in  the  earlier  andesite  was 
encountered  and  followed  in  the  shaft  to  a  depth  of  392  feet,  where  the  first 
mine  level  was  made.  The  other  main  levels  are  at  460,  512,  612,  and  765  feet. 

The  Mizpah  fault  was  cut  in  a  northeast  drift  on  the  392-foot  level,  as  shown 
in  fig.  58,  at  a  point  about  60  feet  from  the  shaft;  it  was  also  encountered  in 
the  512-foot  level,  as  shown  in  PL  XXII.  Its  strike  and  dip  are  therefore  fairly 
well  determined;  the  strike  is  about  N.  55°  W.,  and  the  dip  is  northeast,  at  an 
angle  of  about  29°.  The  later  andesite  has  been  found  on  the  northeast  or  upper 
side  of  this  fault,  at  all  depths  thus  far  examined,  both  in  this  mine  and  in 
neighboring  OUCH. 

This  rock  (the  later  andesite)  has  been  extensively  explored,  both  in  this 
mine  (as  in  the  drift  on  the  512-foot  level  connecting  the  Montana  Tonopah  and 
North  Star  shafts)  and  in  others,  but  no  veins  of  size  and  value  have  been  found, 
nor  anything  that  does  not  confirm  the  theory  that  the  principal  veins  are  older 
than  the  later  andesite. 


MONTANA  TONOPAH  MINE. 


169 


VEIN    ON    THE   392-FOOT    LEVEL. 

The  nature  ana  relations  of  the  Montana  Tonopah  veins  are  best  seen  from 
figures.  Fig.  58  shows  the  upper  or  392-foot  level,  and  the  plan  of  the  vein 
first  encountered  in  the  shaft  at  that  level.  The  vein  is  about  3  feet  thick,  of 
the  normal  Tonopah  type,  such  as  has  resulted  from  a  silieification  and  minerali- 
zation of  the  rock  along  a  zone  of  ^___ 

close-set  fractures;  the  values  in  it  are 
moderate.  It  is  sharply  cut  off  on  the 
east  by  the  Mizpah  fault.  Near  the 
shaft  it  is  cut  by  a  number  of  small 
northeast  faults,  generally  steep  and 
dipping  in  both  directions.  These  faults 
nearly  always  have  brought  about  an 
upthrow  on  the  northwest  side,  so  that 
in  horizontal  plan  the  vein  is  offset  to  the 
southwest  on  the  southeast  side.  These 
faults  are  both  normal  and  reversed 
(fig.  59).  The  vein  dips  northwest  at 
an  average  angle  of  45-  or  50°. 

This  level,  continued  as  a  cross- 
cut about  150  feet  to  the  northeast, 
cuts  another  vein,  supposed  to  be  the 
Macdonald  vein  of  the  lower  levels. 
This  vein  strikes  northeast  and  dips  northwest  at  an  angle  of  40°;  it  is  from  '2 
to  4  feet  thick  and  contains  some  good  ore.  Two  portions  of  it,  separated  by 
a  northeasterly  striking,  southeast  dipping  (60°)  fault,  are  successively  cut  in  the 
drift.  On  account  of  this  faulting  the  vein  has  not  been  much  explored. 

Shaft  BRANCH    VEIN    ON    THE   460-FOOT    LEVEL. 

1 

At  440  feet  the  shaft  cuts  a 
minor  vein  below  the  one  iust 
described  (fig.  60).  This  vein  is 
about  4  inches  thick  at  the  shaft 
and  was  followed  a  short  distance 
northeast  along  its  strike.  At  a  distance  of  about  25  feet  it  was  represented  only  by 
stringers  2  inches  or  less  thick,  and  was  not  farther  drifted  upon.  To  the  southwest 
of  the  shaft,  what  is  probably  the  same  vein  was  followed  a  longer  distance, 
becoming  stronger  and  being  from  8  to  18  inches  thick.  The  ore  in  this  part  of 
the  vein  is  often  of  high  grade,  consisting  of  black  and  white  quartz,  crustified  or 


FIG.  58. — Horizontal  plan  of  faults  and  vein  on  the  392-foot 
level  of  the  Montana  Tonopah. 


zo  feet 


Flo.  69.— Vertical  section  along  north  drift.  392-foot  level,  Montana 
Tonopah. 


170 


GEOLOGY    OF   TONOPAH   MINING    DISTRICT,   NEVADA. 


irregularly  mingled.     The   black   quartz    owes    its   color    to    a    large    amount   of 
included  black  silver  sulphide  and  other  sulphides. 

In  this  drift  southwest  of  the  shaft  the  vein  dips  to  the  northwest  at  an 
angle  of  about  60°. 

CONNECTION   OF   BRANCH    VEIN    WITH    MONTANA    VEIN. 

This  vein  was  followed  southwest  on  the  strike  and  downward  on  the  dip 
to  its  junction  with  a  larger  and  more  important  vein — the  Montana.  At  a 
point  a  little  over  40  feet  southwest  of  the  shaft,  an  incline  on  the  vein  went 
down  38  feet  to  the  Montana  vein,  while  the  same  junction  along  the  strike  was 
effected  at  a  point  over  100  feet  southwest  of  the  shaft.  The  Montana  vein 


o       PO      20     30      40     so  feet 


Pie.  60.— Horizontal  plan  snowing  veins  and  faults  on  the  100-foot  level  of  the  Montana  Tonopali. 

strikes  at  this  point  generally  east  and  west,  and  dips  north  at  an  angle  of  45^  or 
55°,  the  dip  being  somewhat  less  than  that  of  the  smaller  vein.  The  junction  of 
the  two  veins  therefore  pitches  to  the  northeast  at  a  comparatively  low  angle. 


HHKCC1ATED   STRt'CTUKE    IN    THE    MONTANA    VEIN. 


The  Montana  vein  as  developed  in  this  level  was  very  strong.  It  was  from  6 
to  8  feet  thick,  being  rather  thicker  than  the  average  vein.  It  showed  white 
quartz  with  dark-colored  portions  and  had  often  a  brecciated  structure.  The 
dark  quartz,  which  contains  a  much  larger  amount  of  black  silver  sulphides  than 
the  light-colored  quartz,  proves  on  assay  to  contain  three  times  or  more  the 
value  of  the  white. 


MONTANA  TONOPAH  MINE. 


171 


Examination  of  the  breccia  shows  that  frequently  the  black  quartz  occurs  as 
angular  fragments  cemented  by  the  white,  while  in  other  places,  perhaps  in  the 
same  exposure  of  the  vein,  fragments  of  the  white  quartz  are  cemented  by  the 
darker  and  richer  ore.  The  whole  is  a  solid,  substantial  vein,  both  dark  and 
white  quartz  having  every  mark  of  primary  origin.  The  only  trace  of  movement 
is  in  the  brecciation  of  the  dark  and  white  quartz,  as  above  described. 


PIG.  61. — Figure  drawn  from  sketch,  showing  face  of  ore  of  the  Montana  vein  on  the  west  drift,  460-foot  level, 
Montana  Tonopah  mine.  To  illustrate  fissure  with  crustified  high-grade  ores,  subsequent  to  the  formation  of  the 
ordinary  veins,  but  within  the  period  of  primary  ore  deposition;  a,  altered  andesite,  wall  rock;  b,  main  Montana 
vein  of  ordinary  type;  c,  subsequent  fissure  filling.  Within  this  the  dark  streaks  are  rich  black  sulphide  layers, 
with  quartz  and  carbonates  between.  In  the  central  quartz  band  are  druses  lined  with  adularia  crystals. 

CBD8TIPICATION    IN    THE    MONTANA    VEIN. 

Another  allied  peculiarity  of  this  vein,  as  compared  with  other  veins  of  the, 
district,  is  that  portions  are  regularly  banded  or  crustified. 

Such  crustification  is  not  characteristic  of  the  whole  vein,  for  the  crustified 
portion  occurs  surrounded  by  solid  quartz  possessing  no  banded  or  comb  structure 


172  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 

and  having  all  the  characteristics  of  the  typical  quartz  vein  of  the  district.  The 
trustified  vein  also  is  not  regular  nor  persistent,  and  seems  to  have  filled  uneven 
clefts  or  openings  in  the  main  vein,  which  itself  has  every  appearance  of  having 
been  formed  by  silicification  along  fracture  zones  in  the  way  previously  outlined 
for  the  outcropping  veins  of  Mizpah  Hill  (fig.  61). 

CONDITIONS   OF    KOKMATION    OF    MONTANA    VEIN. 

The  gangue  and  the  metallic  contents  of  the  crustified  vein  are,  however,  of 
exactly  the  same  kind  as  those  of  the  ordinary  inclosing  vein.  There  is  no 
reason  to  doubt  that  both  portions  of  the  vein  are  primary,  like  the  different 
depositions  noted  in  the  breccia  ore.  The  phenomena  indicate  that,  in  this  portion 
of  the  vein  at  least,  rock  movement  went  on  subsequent  to  the  first  ore  depo- 
sition and  to  the  first  cementation  of  the  fractures  by  quartz,  producing  in  places 
a  breccia,  which  was  cemented  with  similar  materials  by  vigorously  circulating 
mineralizing  waters,  and  even  forming  irregular  open  spaces,  in  which  the  ores 
and  gangue  materials  were  deposited  in  successive  layers.  It  seems  that  the 
movement  continued  even  after  the  beginning  of  the  deposition  of  some  of  these 
crustified  masses,  for  some  of  the  breccia  ores  show  fragments  of  very  light  and 
of  very  black  quartz,  such  as  are  characteristic  of  the  crustified  veins  and  not  of 
the  ordinary  type,  intimately  associated.  The  later  part  of  the  mineralization 
thus  indicated  may  have  occurred  at  a  period  when  the  solutions  were  richer  in  the 
metallic  minerals  than  previously,  for  this  portion  of  the  vein  is  characterized 
bv  extremely  rich  ore,  and  some  of  the  faces  exposed  in  breaking  down  the  vein 
showed  great  masses  of  the  black  sulphides,  constituting  ore  of  a  richness  that 
is  rarely  seen  in  such  quantity. 

KATLT8    ON    THE   460-FOOT    LEVEL. 

As  shown  on  fig.  60,  the  northeast  branch  vein  is  interrupted  by  a  number  of 
minor  slips  or  faults.  On  the  east  the  Montana  vein  is  sharply  cut  by  northeast 
faults  having  a  southeast  dip  of  about  35°,  and  its  eastward  continuation  has  not 
been  found.  The  smaller  faults  of  this  series  show  that  the  result  as  seen  in 
horizontal  section  is  an  offset  to  the  south  on  the  east  side.  Such  an  effect  might  be 
due  to  a  variety  of  displacements;  in  this  case  the  strong  striae,  pitching  east  at  an 
angle  of  30°  on  the  fault  planes,  show  a  diagonal  downthrow  on  the  east.  According 
to  this,  the  continuation  of  the  Montana  vein  should  be  offset  to  the  south  from  the 
present  course. 

The  relative  positions  of  the  Montana  and  Mizpah  veins  at  this  level  are  shown 
in  fig.  62. 


MONTANA    TONOPAH    MINE. 

VEINS    OX    THK    512-FOOT    LEVEL. 


178 


The  Montana  vein  ha.s  been  followed  from  the  460-foot  level  up  to  the  fault  and 
has  been  traced  downward  to  the  512-foot  level.  The  .situation  on  this  level  is  shown 
on  PI.  XXII.  The  vein  marked  on  this  diagram  "Montana  vein"  has  been  shown, 
by  tracing  the  actual  connection,  to  be  the  same  as  the  vein  on  the  460-foot  level. 
On  the  northeast  it  grows  less  strong  and  definite  on  reaching  the  main  north  drift. 

In  the  east  drift  a  cross  wall,  striking  nearly  parallel  with  the  vein,  but  dipping 


460-ft  level^ 
3ft.above400-ft.Mizpah/  ' 


N 


Tonopah  Mining  Co- 
main  shaft 


Mizpah  400-ft  level 


Scale 
100  200 


300  feet 


FIG.  82.— Horizontal  plan  showing  relations  of  the  Mizpah  and  Montana  veins  on  the  400-foot  level  of  the  M  i/pah. 

in  the  opposite  direction  (to  the  south)  constitutes  the  lower  limit  of  this  ore  shoot. 
Above  this  cross  wall  the  ore  has  been  continuously  stoped  out.  Below  it  the  walls 
continue,  and  a  good  deal  of  quartz  is  present,  but  no  rich  ore  has  as  yet  been  found 
below  this  point  on  the  vein  (tig.  63). 

A  long  north  drift  from  the  shaft,  on  the  ol^-foot  level,  discloses  two  veins 
parallel  to  the  Montana.  These  are  shown  in  PI.  XXII.  The  one  nearer  the  shaft 
shows  in  the  drift  a  '2-foot  zone  of  quartz  stringers  with  altered  andesite  between. 


174 


GEOLOGY   OF   TONOPAH    MINING   DISTRICT,   NEVADA. 


This  zone  contains  some  silver  sulphides  and  some  good  ore,  although  it  is  largely 
of  low  grade.     The  one  lying  farthest  north  has  been  called  the  Macdonald  vein. 

The  Macdonald  vein  is  a  strong,  rich  vein  having  a  strike  a  little  north  of 
east,  and  a  northerly  dip  varying  from  45°  to  65°.  It  has  been  extensively 
drifted  on,  on  this  level,  and  has  produced  a  great  deal  of  high-grade  sulphide 

s  ore,  of  the  same  character  as  the  high-class 
ores  of  the  Montana  vein.  It  has  been  fol- 
lowed down  to  the  615-foot  level. 

On  both  these  levels  and  on  the  interven- 
ing stopes  this  vein  shows  a  complex  fault- 
ing, reminding  one  of  the  faulting  that  has 
affected  the  Fraction  vein.  In  a  vertical 
section  such  faults  appear  nearly  parallel  to 
the  vein,  but  curve  and  continually  branch 
and  so  become  now  steeper,  now  flatter  in 


FIG.  63.— Vertical  sketched  cross  section  of  cross  wall 
limiting  chief  ore  shoot  of  Montana  vein  below,  as 
displayed  on  the  512-foot  level  of  the  Montana  Tono- 
pah.  a,  Rich  sulphide  ore,  sloped  out;  6,  silicifled 
andesite,  some  quartz  and  ore,  no  rich  ore;  c,  Earlier 
andesite,  wall  rock. 

dip  than  the  veins  (tigs.  64,  65).  If 
straight  this  faulting  would  be  like 
that  which  has  affected  the  vein  of 
the  North  Star,  but  the  undulations 
of  the  faults  here  in  the  Montana 
Tonopah  produce,  in  vertical  section, 
displacements  of  the  vein  to  the  north 
on  the  under  side  of  the  faults.  The 

FIG.  &J.— Vertical  cross  section    (sketched),    showing    effect  of 

line  of  faulting  is  not  parallel  in  strike       curving  and  branching  faults  on  Mncdonald  vein,  in  slopes 

above  the  615-foot  level  on  the  Montana  Tonopnh. 

or  dip  to  the  vein,  though  it  sometimes 

so  appears  in  vertical  section;  in  fact,  the  flat  portions  of  the  fault  pianos  pitch  east 
on  the  vein  at  moderate  angles;  and  striiv  along  the  faults  show  that  the  real 
direction  of  movement  has  been  to  the  east  along  this  pitch.  In  horizontal  sec- 
tion, however,  these  faults  are  seen  to  curve  and  branch  in  as  complicated  a 
manner  as  in  the  vertical  section,  producing  an  unrivaled  complexity  (PI.  XXII). 


zo  feet 


MONTANA  TONOPAH  MINE. 


EASTERLY    PITCH    OF  ORE   BODIES. 


175 


There  appears  to  be  an  easterly  pitch  to  the  chief  ore  shoot  on  the  Montana 
vein,  as  this  has  been  developed  in  following  down  the  vein  from  the  460-  to  the 
51'2-foot  level.  Some  of  the  richest  ore  in  the  460-foot  level  lies  vertically  over 
a  relatively  poor  part  of  the  512-foot  level,  the  rich  ore  in  the  latter  level  lying- 
farther  east. 

On  the  Macdonald  vein  the  ore  shoots  pitch  to  the  east. 


Ill  I  \         *  /  '  >^  I     — */  I        /       _  X  ^  f       >  '  *     /  /  »V\'r. 

^Ssy;^^^v^x^^^ 

y^";;;/v\'/:V^V  VA'.y,;/^;;,-'/^' ;-C;0-;o^ ,X\\>, N/.^/^v 
;;v-v/;v^y>  ^.^''V/'-'^Av',;';'^;^;^;/,^:^/^/^'/ 
•-''>-. '-;if;  •  5  /vV/V'Vv:^/:^:^';/^'^/.;^;^;';^/-;^ 

:;,<',',^,f\/ /x>\vs>",y^  ^;,;',s;^>;'S,;\',;, ',*:^ 

>;.1",V';y^    V^^^'V^,v;';vvXv:/:::vy/>v>^;''^: 

'\,'c,',';:^-  •-<_/" /,\/>V^Vr\''/sv>/^\\'^/  v^'/oVy'^X'>/%V  ''• 


Scale 
ao  30 


50  feet 


Fiii.  6S.— Vertical  cross  M^ction  (sketched),  showing  effect  of  curving  and  branching  faults  ou  Macdonald  vein,  in  stoi>v» 

above  the  615-foot  level  on  the  Montana  Tonopah. 

TONOPAH    RHYOLITE-DACITE    IN   THE   MONTANA    TONOPAH. 

At  a  depth  of  560  feet  the  Montana  Tonopah  shaft  passed  downward  from 
the  ordinarv  earlier  andesite  which  contains  the  veins  to  a  dense  rock,  which 
proves  to  be  the  glassy  Tonopah  rhyolite-dacite. 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 


The  same  rock  was  encountered  at  the  bottom  of  a  winze,  2<>  feet  below  the 
west  drift  on  the  ;">  12-foot  level,  where  it  seem*  to  cut  off  the  Montana  vein.     It 


Montana 


-YAx  v  / « -  ->  '/rAffi^£^^\^^^.fj;-,V'/ 

v-#  '  -  ^^^8i^SS 
-          isl^iliM 


300  feet 


Kio.  66.— t'ross  wction  showing  guolofry  exposed  liy  Montaim  Tonopuh  workings. 

is  also  found  on  the  615-  and  765-foot  levels  (fig.  66).  As  is  usual  in  thi.s  formation 
irregular  and  bunchy  quart/  veins  are  encountered,  which  sometimes  yield  good 
assays,  especially  in  gold;  but  no  pay  ore  has  yet  been  found. 


MONTANA    TONOPAH    VEIN    SYSTEM. 


177 


NORTH    STAR   WORKINGS. 


SECTION    PASSED  THROUGH. 


The  North  Star  shaft  was  started  in  white  rhyolite  on  the  slope  of  Mount 
Oddie  (fig.  67).  Below  the  rhyolite  comes  the  later  andesite,  the  contact  being 
practically  horizontal  and  indicating  the  later  age  of  the  rhyolite.  From  this 
contact  down  to  a  depth  of  about  720  feet  the  shaft  is  in  the  later  andesite, 
largely  soft  and  decomposed.  It  is  sometimes  brecciated,  indicating  considerable 
movement,  and  in  places  contains  much  secondary  pyrite.  At  depths  of  about 


KIG.  07.— Section  on  plane  of  Desert  Queen  and  North  Star  shafts. 

720  to  740  feet  the  shaft  cuts  the  zone  of  the  Mizpah  fault,  which  is  characterized 
by  20  feet  or  more  of  clay,  formed  by  trituration  and  decomposition  along  the 
fault.  Beneath  the  fault  the  earlier  andesite  comes  in. 

Just  above  the  bottom  of  the  shaft,  which  is  1,050  feet  deep,  the  Tonopah 
rhyolite-dacite  comes  in.  It  is  the  same  sheet  which  is  encountered  on  the  814- 
foot  level  of  the  Desert  Queen. 

The  developments  in  the  North  Star  consist  of  two  drifts,  at  950  and  1,050  feet. 
The  lower  one  of  these  levels  is  the  more  extensive,  having  a  drift  to  the  north 
of  over  500  feet.  On  the  950-foot  level  and  on  the  north  drift  of  the  1,050-foot 
16843— No.  42—05 12 


178  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 

level  the  rock  is  andesite,  probably  earlier  andesite,  largely  altered  to  chlorite 
and  calcite,  like  that  below  the  Siebert  fault  in  the  Mizpah  shaft.  The  station  at 
the  1,050-foot  level  and  a  drift  running  southeastward  from  the  shaft  for  over  200 
feet  are  mainly  in  the  Tonopah  rhyolite-dacite.  This  rock  is  much  silicified  and 
is  in  places  cherty  quartz.  At  the  shaft  and  on  the  walls  of  the  drift  in  this 
formation  there  has  formed,  since  the  opening  of  the  mine,  a  green  coating.  This 
was  determined  by  Dr.  W.  T.  Schaller,  of,  the  United  States  Geological  Survey, 
to  be  a  basic  copper  sulphate,  insoluble  in  water.  The  cherty  quartz  on  which 
this  incrustation  forms  contains  only  traces  of  gold  and  silver.  Since  the  mine  is 
perfectly  dry  the  formation  of  this  copper  sulphate  on  the  walls  is  interesting. 
A  similar  incrustation  forms  on  the  quartz  of  the  rhyolitic  "veins  on  the  84rO-foot 
level  of  the  Desert  Queen.  It  seems,  so  far  as  observed,  to  be  a  phenomenon 
peculiar  to  the  quartz  of  the  Tonopah  rhyolite-dacite  and  to  have  no  connection 
with  gold  and  silver  values. 

On  the.- 1,050- foot  level  in  the  earlier  andesite  a  phenomenon  was  noted  which 
was  not  observed  elsewhere1  in  the  camp.  This  is  the  intrusion  of  one  body  of . 
earlier  andesite  by  another  body  of  the  same  rock.  The  intrusive  rock  is  tiner 
grained  than  the  rock  which  it  cut,  and  near  the  margin  showed  flow  structure. 
The  coarser  intruded  rock  is  of  the  biotite-bearing  variety,  while  the  intrusive 
rock  is  of  .v.ery  similar  composition  and  is  very  typical  earlier  andesite.  This 
occurrence  is  analogous  to  the  finding  in  the  Tonopah  City  shaft  of  dikes  of 
Heller  dacite  intrusive  into  a  body  of  the  same  rock,  and  signifies  successive  injec- 
tions of  the  earlier  andesite,  which  may  very  well  be  of  slightly  different  types 
as  regards  composition. 

VEINS. 

On  the  950  foot  level,  north  of  the  shaft,  a  vein  of  quartz  several  feet  thick 
was  cut  in  the  earlier  andesite.  This  has  a  general  west-northwest  strike  and 
a  northerly  dip  of  45°  or  50°.  This  vein  was  cut  also  in  the  1,050-foot  level 
and  is  developed  by  an  incline  between  the  two  levels.  Some  ore  has  been  shipped 
from  it,  having  the  same  characteristics  as  the  ore  of  the  Montana  Tonopah;  it 
contains  polybasite,  ruby  silver,  etc.,  in  a  white  quartz  gangue.  This  is  very 
likely  the  same  as  the  Montana  vein  of  the  Montana  Tonopah. 

FAULTING. 

It  has  not  been  possible  to  follow  this  vein  very  far  along  the  strike  or  dip 
in  any  one  place  on  account  of  faulting,  which  follows  the  vein  very  nearly  in 
strike  and  dip  but  curves  and  becomes  oblique  to  it.  On  the  950-foot  level  this 
fault  nan  a  strike  whicli  is  more  northerly  thitn  that  of  the  vein,  and  so  has  cut 
out  most  of  the  vein,  leaving  only  a  wedge.  On  this  level  the  fault  is  below  the 
vein,  bufc  in  following  the  incline  down  to  the  level  below,  it  is  found  to  puss 
through  the  vein  and  to  go  into  the  hanging  wall,  us  shown  in  fig.  67. 


MONTANA   TONOPAH   VEIN   SYSTEM.  179 

MIDWAY    WORKINGS. 

The  Midway  lies  a  short  distance  northwest  of  thte  Siebert  shaft,  and 
almost  in  line  with  and  halfway  between  the  Montana  Tonopah  and  the  Tonopah 
Extension. 

LATER   ANDESITE   IV  SHAFT. 

The  surface  at  this  point  is  composed  of  tlie  typical  later  andesite.  A 
specimen  taken  a  short  distance  from  the  Midway  shaft  has  the  characteristic 
relatively  fresh  appearance,  dark  color,  and  large  feldspars  of  this  rock.  Under 
the  microscope  it  is  also  typical,  showing  numerous  phenocrysts  crowded 
together,  these  phenocrysts  being  mainly  feldspars,  often  large  and  compound, 
with  pseudomorphs  of  serpentine  after  pyroxene. 

The  contact  of  this  rock  with  the  underlying  earlier  andesite  is  an  obscure 
one.  This  is  a  condition  similar  to  that  noted  in  other  workings,  such  as  the 
West  End  and  MacNamara,  where,  as  described,  the  contact  between  the  two 
andesites  could  not  be  located  in  the  shafts. 

TYPICAL    EARLIER    ANDE8ITE    IN    SHAFT. 

In  the  case  of  the  Midway,  as  shown  in  the  section  (fig.  68),  the  contact 
has  been  perhaps  rather  arbitrarily  drawn  at  a  depth  of  about  425  feet.  From 
this  point  to  a  point  just  below  475  feet  in  the  shaft  the  formation  is  regarded  as 
probably  all  typical  earlier  andesite. 

GLASSY     TONOPAH     RHYOLITE-DACITE    IN     SHAFT. 

At  a  point  in  the  shaft  below  475  feet  there  is  a  change  in  the  formation, 
and  the  rock  is  quite  uniform  and  of  the  same  hard,  siliceous  nature  and  light- 
green  color  as  that  at  the  main  level  of  the  Ohio  Tonopah. 

This  rock  contains  jaspery  quartz  veinlets  and  fine  quartz  lines  some  of  the 
cavities  left  by  the  removal  of  pyrite  and  other  crystals. 

FORMATIONS    EXPOSED    BY    DRIFTING. 

The  workings  of  the  Midway  consist  of  two  levels  at  depths  of  535  and  685 
feet,  the  former  having  a  north  drift  over  400  feet  long  and  a  south  drift  about 
150  feet  long,  while  the  latter  has  a  north  drift  nearly  700  feet  long  and  a  south 
drift  of  about  150  feet.  The  formation  in  the  upper  level  is  entirely  Tonopah 
rhyolite-dacite,  except  at  the  end  of  the  north  drift,  which  passes  through  the 
same  contact  as  that  encountered  in  the  shaft  and  enters  the  earlier  andesite. 
The  shaft  passes  downward  through  the  body  of  rhyolite-dacite  and  enters  earlier 
andesite  beneath  it,  of  a  type  like  that  found  on  the  700-foot  level  of  the  Siebert 
shaft.  Similar  andesite  is  encountered  on  the  south  drift  of  the  (>35-foot  level, 
while  the  whole  of  the  north  drift  on  this  level  lies  in  the  rhvolite-dacite. 


180  GEOLOGY    OF   TONOPAH    MINING    DI8TBICT,   NEVADA. 

VEINS   IN   THE   MIDWAY. 

There  are  veinlets  of  calcite  in  the  later  andesite  and  these  very  often  contain 
pyrite.  In  the  shaft  at  a  depth  of  about  -430  feet  there  are  quartz  stringers 
containing  pyrite.  At  a  depth  of  435  feet  there  is  a  short  northwest  drift,  showing 
a  vein  of  black,  jaspery  quartz,  which  is  barren  and  irregular. 

A  fragment  of  a  vein  was  cut  at  475  feet  in  the  shaft.  The  vein  was  largely 
barren  but  contained  a  rich  bunch  or  shoot  of  original  sulphide  ore.  This  ore. 


Flo.  fi«._ Section  showing  geology  exposed  by  Midway  wiirkint-s. 

when  examined  microscopically,  shows  the  typical  structure  of  the  productive 
earlier  andesite  veins.  The  quartz  has  the  usual  varied  grain,  ranging  from 
inicrocrystalline  to  medium  crystalline.  There  is  scattered  pyrite  seeming  to  have 
no  relation  to  the  values,  which  consist  of  black  silver  sulphide  and  silver  chloride, 
both  of  which  are  relatively  abundant.  The  relation  between  these  two  is  remark- 
able, for  the  black  sulphide  forms  rims  around  the  chloride  and  in  some  cases  is 
found  along  cracks,  showing  that  it  was  formed  later  than  the  chloride  and  is 
very  probably  an  alteration  product  of  it.  There  is  occasionally  a  little  ruby  silver, 


TONOPAH    EXTENSION    MINE.  181 

having  the  same  relation  to  the  chloride  as  does  the  sulphide.     This  black  sulphide 
may  be  either  argentite  or  stephanite. 

This  quartz  vein  is  much  broken,  so  that  the  general  strike  and  dip  could  not 
l)e  determined.  It  may  very  well  be  the  extension  of  one  of  the  veins  developed 
in  the  Montana  Tonopah." 

As  usual  in  other  parts  of  the  camp,  the  Tonopah  rhyolite-dacite  in  the 
Midway  contains  a  number  of  quartz  veins  which,  however,  are  irregular,  non- 
persistent,  and  faulted,  and  are  usually  barren.  The  most  important  vein  of  this 
class  was  encountered  on  the  535-foot  level,  a  short  distance  south  of  the  shaft. 
This  shows  several  feet  of  quartz,  striking  in  a  west-northwest  direction  and  having 
a  steep  dip.  On  the  southeast  this  vein  becomes  irregular  and  passes  into  barren, 
cherty  quartz,  which  in  turn  disappears,  turning  to  silicitied  rhyolite-dacite. 
Most  of  the  vein  is  barren,  but  at  one  point  400  tons  of  ore,  having  a  value  of 
$30  to  the  ton,  were  taken  out  and  milled.  A  winze  follows  this  vein  to  the  lower 
contact  of  the  rhyolite-dacite  with  the  earlier  andesite,  a  short  distance  above  the 
635-foot  level.  The  silicitication  and  the  vein,  however,  cease  at  the  rhyolite- 
dacite  contact  and  do  not  enter  the  earlier  andesite,  into  which  the  rhyolite-dacite 
is  intrusive. 

A  second  vein,  having  an  east-west  course,  was  encountered  about  50  feet  north 
of  the  shaft  on  the  535-foot  level.  It  dips  about  65°  to  the  south  and  is  termi- 
nated on  the  east  by  a  fault,  so  far  as  explored.  It  is  about  2-  feet  in  thickness 
and  contains  a  little  good  ore,  of  which  a  few  tons  have  been  shipped;  the  rest 
of  the  vein  is  barren.  About  150  feet  north  of  this  last-named  vein,  on  the  same 
level,  there  is  smother  2-foot  vein  of  white  barren  quartz,  which  has  a  west-south- 
west strike  and  a"  northerly  dip  of  80°.  This  contains  no  ore,  but  only  white, 
barren  quartz,  although  assays  of  from  $20  to  $30  can  be  had. 

That  these  veins  are  nonpersistent  is  shown  not  only  by  the  developments 
upon  this  level,  but  by  the  fact  that  they  are  not  found  in  the  same  formation 
on  the  635-foot  level  100  feet  below.  Although  this  level  runs  through  650  feet 
of  rhyolite-dacite  it  encounters  no  strong  and  definite  veins. 

TONOPAH    EXTENSION     MINE. 
CONTACT    OF    EARLIER    AND    LATER    ANDESITE8. 

The  Tonopah  Extension  shaft  starts  in  the  later  andesite  and  extends  down  about 
183  feet  to  the  contact  of  the  earlier  andesite  (see  fig.  71).  This  contact  is  marked 
by  1  to  2  feet  of  soft,  decomposed  rock,  and  is  very  flat.  Below  it  the  earlier 
andesite  is  ^  ory  full  of  quartz  veinlets.  This  phase  of  the  earlier  andesite  resem- 
bles in  many  places  some  of  the  phases  of  the  later  andesite,  although  just  below 

aSince  the  writer's  visit  more  ore  hu.s  been  fouud'in  the  Midway,  in  a  drift  at  about  this  level. 


182 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 


the  contact  above  referred  to  it  is  fairly  typical.  The  contact  is  probably  not 
due  to  faulting,  but  is  normal  and  indicates  that  the  veins  in  the  earlier  andesite 
outcropped  at  the  surface  at  the  time  of  the  later  andesite  extrusion. 


VEINS    IN    THE    EARLIER    ANDESITE. 


At  a  depth  of  230  feet,  in  the  earlier  andesite,  a  heavy  vein  was  cut  near  the 
shaft.  This  has  been  developed  by  levels  at  depths  of  244  and  385  feet,  and  by  an 
incline  between  the  levels.  The  general  strike  of  the  vein  is  west-northwest  and 
the  dip  north  from  30°  to  45°.  The  vein  is  from  3  to  8  feet  thick  and  shows  shoots 
of  high-grade  sulphide  ore  like  that  of  the  Montana  Tonopah.  So  far  as  had  been 
developed  at  the  time  of  the  writer's  visit,  in  November.  1904,  the  vein  has  not 
been  faulted. 


tofeet 


Fio.  69.— Diagrammatic  vertical  cross  section  of  Tonopah  Extension  vein.  a.  Altered  earlier  andesite,  wall  rock;  6, 
typical  white  vein  of  earlier  andesite  period,  containing  black  silver  sulphides,  with  values  of  several  hundred  dollars 
per  ton;  c,  black,  jaspery  quartz  of  later  introduction  than  original  vein,  of  which  it  contains  fragments.  Values  of 
black  quartz  and  fragments,  820  to  $30  per  ton. 

An  interesting  phenomenon  is  displayed  by  the  Tonopah  Extension  vein.  Where- 
ever  it  has  been  followed,  a  portion  of  the  vein,  generally  that  lying  next  to  the 
hanging  wall,  is  of  different  character  from  the  rest.  The  main  body  is  composed 
of  white  quartz  containing  black  silver  sulphides,  and  has  exactly  the  same  char- 
acter as  the  other  earlier  andesite  veins  in  the  camp.  The  upper  portion,  however, 
is  of  black  or  gray  jaspery  quartz,  like  so  many  of  the  veins  in  the  Tonopah 
rhyolite-dacite.  Moreover,  this  portion  contains  angular  fragments  of  the  ordinary 
quartz  vein  in  such  a  way  as  to  show  conclusively  that  the  jaspery  quartz  was  of 
later  introduction  than  the  main  vein.  Evidently  renewed  pressure  reopened  the 
vein  subsequent  to  the  first  ore  deposition,  and  caused  a  new  fracture  or  fissure, 


TONOPAH    EXTENSION    MINE. 


183 


following  in  general  the  old  hanging  wall.  Along  this  opening  waters  have  cir- 
culated and  deposited  jaspery  quartz,  cementing  the  broken  fragments  of  the  old 
vein.  On  the  244-foot  level  the  thickness  of  the  jaspery  subsequent  quartz  is  about 
li  feet,  while  that  of  the  typical  antecedent  quartz  is  about  3  feet.  At  the  place 
where  the  sketch  (tig.  69)  was  made,  the  lower  part  has  a  value  of  about  $600,  while 
the  jaspery  quartz  has  values  of  from  $30  to  $35.  Moreover,  it  is  probable  that 
these  last-named  values  are  in  large  part  derived  from  included  fragments  of  the 
true  vein,  and  also  from  the  ruby  silver  which  is  sometimes  found  in  cracks  in  the 
jaspery  quartz  as  well  as  in  the  true  vein,  this  ruby  silver  being  a  secondary,  mineral 
derived  from  the  primary  ore. 

The  general  character  of  this  subsequent  vein  filling  renders  it  highly  probable 
that  this  vein  is  of  the  same  nature  and  period  as  the  veins  in  the  Tonopah 
rhyolite-dacite.  While  the  main  vein  was  formed  after  the  eruption  of  the  earlier 


O  Midway  shaft 


North  Star  shaft 

5t? 
•j<*  North  Star  vein  [projected) 

Wl°°13'    *  Montana  Tonopah  shaft 


^TONOPAH 
Extension  shall 


Mac  Namara  shaft 


TONOPAH  MINING  CO 

Main  shaft 


Mizpah  vein 


•J       * 

V 


Belmont  vein 


•  West  End  shaft 


Valley  View  shaft 


Scale 
500 


Fni.  70. — Map  showing  principal  earlier  andesite  veins  now  developed  undergound,  within  the  main  productive  area: 
shown  on  the  horizontal  plane  of  the  Mizpah  500-foot  level. 

andesite,  the  subsequent  tilling  took  place  after  the  eruption  of  the  rhyolite-dacite. 
This  main  vein  in  the  Tonopah  Extension  is  probably  identical  with  one  of  the 
veins  in  the  Mizpah  or  the  Montana  Tonopah.  Very  possibly  it  is  the  same  as  the 
Montana  vein,  but  this  can  not  be  definitely  proved  as  yet. 

VEINS   IN   THE  TONOPAH    RHYOLITE-DACITE. 

The  above  conclusions  as  to  subsequent  filling  are  strengthened  by  certain 
other  occurrences  in  this  same  mine.  On  the  385-foot  level  a  south  drift  from 
the  shaft  cut  the  upper  contact  of  a  flat- lying  north -dipping  body  of  Tonopah 
rhyolite-dacite.  In  this  last-named  rock,  near  the  contact  with  the  earlier  andesite, 
there  is  a  great  deal  of  silicification,  amounting  often  to  the  formation  of  bodies 
of  pure  jaspery  quartz,  of  very  irregular  size  and  extent,  and  practically  barren 


184  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 

for  the  most  part.  The  main  shaft  passes  through  this  contact  between  the 
385-foot  level  and  the  bottom,  which  is  at  a  depth  of  485  feet,  and  from  the 
bottom  a  north  drift  runs  out  about  100  feet  to  the  contact  again.  The  heavy 
silicitication  resulting  in  the  formation  of  jaspery  barren  quartz,  especially  near 
the  contact,  is  shown  also  on  this  level. 

This  contact  was  followed  upward  from  the  385-foot  level  by  means  of  an 
incline  for  some  distance,  and  showed  more  or  less  of  the  same  rhyolitic  quartz. 
The  dip  of  this  silicified  contact  is  less  than  that  of  the  Tonopah  Extension  vein 
in  the  earlier  andesite,  so  that  very  likely  these  may  come  together  at  a  greater 
depth,  in  which  case  the  barren  jaspery  portion  of  the  Tonopah  Extension  vein 
will  unite  with  the  similar  quartz  in  the  rhyolite -dacite,  with  which  it  has 
undoubtedly  a  common  origin.  In  this  eventuality,  however,  the  productive 
portion  of  the  Tonopah  Extension  vein  may  be  cut  off. 

The  relative  position  of  the  Tonopah  Extension  vein  in  regard  to  that  of 
other  known  veins  of  similar  character  is  shown  in  fig.  70. 

OTHER  EXPLORATORY  WORKINGS  WHOLLY  OR  PARTLY  IN    EARLIER 

ANDESITE. 

WEST   END  WORKINGS. 
OUTCROP  OF   WEST   END    FAULT. 

As  the  map  (PI.  XVI)  shows,  the  West  End  shaft  is  near  the  contact  of  the 
Fraction  dacite  breccia  on  the  southwest  and  the  later  andesite  on  the  northeast. 
This  contact  follows  a  straight  line,  and  was  judged,  from  a  study  of  the  surface 
only,  to  be  due  to  faulting.  By  projecting  the  known  outcrops  of  the  Gold  Hill, 
and  the  Wandering  Boy  faults  it  is  seen  that  they  would  normally  come  together 
in  the  vicinity  of  the  West  End  shaft.  Here  they  probably  unite  to  form  a 
fault  which  is  a  direct  continuation  of  the  Gold  Hill  fault,  and  which  is  thought 
to  have  been  recognized  farther  on,  in  the  line  separating  the  later  andesite 
from  the  Fraction  dacite  breccia,  in  the  vicinity  of  the  MacNamara  shaft.  This 
united  fault  may  be  called  the  West  End  fault.  In  general  this  fault  appears  to  be 
downthrown  on  the  southwest,  for  the  Fraction  dacite  breccia  on  this  side  is 
younger  than  the  later  andesite  on  the  northeast.  Moreover,  both  the  Gold 
Hill  and  the  Wandering  Boy  faults  are  downthrown  on  the  southwest  side. 

RHYOLITE   INTRUSION    ALONG    FAULT. 

Near  the  West  End  shaft  are  seen  rugged  outcrops  of  dark-weathering 
rhyolite,  which  belong  to  a  dike  or  neck  of  rhyolite  that  has  ascended  along  the 
fault  plane.  Where  encountered  in  the  mine  workings  this  rhyolite  is  white, 
and  of  the  same  type  as  the  rhyolite  of  Mount  Oddie,  and  is  probably  of  the  same 
age  and  origin. 


WEST    END    WORKINGS.  185 

The  West  End  shaft  when  last  visited  by  the  writer  was  780  feet  deep. 
The  soft  Fraction  dacite,  which  forms  the  block  on  the  southwest  side  of  the 
fault,  is  first  encountered  in  the  shaft,  but  at  a  depth  of  about  20  feet  the  Oddie 
rhyolite  comes  in.  The  contact  of  dacite  and  rhyolite  strikes  N.  35°  to  55e  W., 
or  roughly  parallel  with  the  West  End  fault,  and  the  dip  is  southwest  at  an 
angle  of  aboijt  65°,  suggesting  that  the  fault  also  dips  in  this  direction  and  is 
therefore  normal.  The  contact  is  partly  tight  and  partly  separated  by  several 
feet  of  breccia,  containing  fragments  of  rhyolite  and  of  later  andesite,  with  the 
soft  materials  of  the  more  fragile  dacite.  The  rhyolite  contact  conies  in  on 
the  north  side  of  the  shaft  and  continues  straight  down  to  a  depth  of  about 
62  feet,  where  it  passes  out  on  the  south  side.  The  general  dip  of  the  rhyolite 
dike  is  therefore  to  the  south.  At  one  or  two  places  the  rhyolite  is  evidently 
intrusive  into  the  dacite.  The  shaft  passes  downward  through  the  upper  contact 
of  the  rhyolite  with  the  breccia  and  traverses  solid  rhyolite  for  a  short  distance, 
showing  that  here  the  thickness  of  the  dike  or  neck  is  about  20  feet.  On  the 
under  contact  of  the  rhyolite,  at  a  depth  of  84  feet,  is  green  altered  andesite, 
which  has  been  referred  to  the  later  andesite.  At  this  contact  also  there  is  a 
slight  breccia. 

The  above  phenomena  are  interpreted  as  indicating  that  the  rhyolite  ascended  • 
along  a  fault  plane,  which  in  the  upper  part  of  the  shaft  separates  the  Fraction 
dacite  from  the  later  andesite.  The  intrusion  of  this  rhyolite  caused  some  brec- 
ciation  of  the  rigid  intruded  rocks  near  the  contact,  and  it  is  possible  that  some 
subsequent  slipping  along  the  fault  may  have  slightly  brecciated  the  rhyolite  itself. 
As  a  rule,  however,  it  has  been  ascertained  that  rhyolite  of  this  sort  is  younger 
than  the  faults  and  is  little  or  not  at  all  affected  by  them. 

At  a  depth  of  116  feet  there  is  a  zone  of  great  movement  and  probable 
faulting,  in  which  the  chief  slips  strike  N.  10°  W.  and  dip  west  at  an  angle  of  25°. 
This  suggests  a  northwesterly  faulting. 

CHARACTER   OF   ANDESITE   ABOVE   220-FOOT   LEVEL. 

Below  the  lower  rhyolite  contact,  at  a  depth  of  84  feet,  the  shaft  is  in 
andesite  for  some  distance.  All  this  andesite  is  extremely  decomposed  in  conse- 
quence of  the  proximity  of  faulting,  and  is  therefore  difficult  to  study.  Below  a 
distance  of  perhaps  100  feet  from  the  surface  the  character  of  the  andesite  has 
occasioned  much  perplexity  in  the  mind  of  the  writer.  The  earlier  and  the  later 
andesites  are  so  closely  related  that  many  times  they  have  almost  identical  char- 
acteristics, and  it  is  difficult  or  impossible  to  discriminate  them  in  the  hand 
specimen  or  under  the  microscope.  A  specimen  taken  in  the  shaft,  at  a  depth  of 
116  feet,  was  judged  to  have  the  characteristics  of  the  later  andesite  rather  than 
of  the  earlier  andesite.  Another  specimen  taken  in  the  shaft,  at  a  depth  of  19t> 


180  GEOLOGY    OF    TONOPAH    MINING     DISTRICT,    NEVADA. 

feet,  was  supposed  to  represent  the  same  rock,  for  no  sharp  division  had  been 
noted,  but  was  judged,  after  microscopic  study,  to  have  rather  the  characteristics 
of  the  earlier  andesite.  This  specimen  was  altered  to  quartz,  sericite.  and  pyrite. 

CHARACTER   OF   ANDESITE    ON    220-FOOT   LEVEL. 

At  a  depth  of  220  feet  from  the  surface,  drifts  were  run  338  feet  to  the  north 
of  the  shaft  and  285  feet  to  "the  south.  In  both  these  drifts  only  andesite  was 
encountered  and  no  general  distinction  was  noted  between  the  andesite  in  the 
different  parts  of  the  drifts.  In  both  drifts  the  rock  strongly  resembles  certain 
phases  of  the  earlier  andesite;  in  the  south  drift  perhaps  more  than  in  the 
north.  This  resemblance  also  holds  good  on  microscopic  study.  Some  sections 
of  the  rock  in  the  north  drift  showed  occasional  original  phenocrysts  of  quartz, 
such  as  are  occasionally  found  in  the  earlier  andesite.  This  original  quartz  was 
found  also  in  the  specimen  obtained  in  the  shaft  at  a  depth  of  196  feet.  On  both 
these  drifts  there  was  evidence  of  considerable  movement,  the  general  strike  of 
the  slip  or  fracture  planes  being  north  and  south  and  the  dip  west  rather  steeply. 
The  andesite  when  examined  microscopically  was  found  to  be  highly  altered,  the 
chief  alteration  products  being  quartz,  calcite,  chlorite,  serpentine,  pyrite.  siderite. 
kaolin,  and  adularia. 

COKKELATION   OF  ANDESITES   IN    WEST    END   AND   FRACTION    WORKINGS. 

After  studying  the  delicate  question  as  to  whether  this  rock  is  the  earlier  or 
the  later  andesite  the  writer  has  satisfied  himself  that  the  andesite  of  the  south 
drift  in  the  West  End  is  identical  with  that  shown  in  the  long  north  drift  from 
the  -100-foot  level  of  the  Fraction  No.  2  shaft.  The  faces  of  the  two  drifts  are 
only  about  250  feet  apart  in  a  straight  line,  but  there  may  be,  and  very  likely  is. 
intervening  faulting.  The  writer  was  not  able  to  distinguish  between  the  general 
type  of  the  andesite  in  this  north  drift  of  the  Fraction  and  the  typical  Fraction 
andesite,  which  is  often  relatively  dark  and  chloritic.  In  the  Fraction  No.  1 
workings  the  andesite  contains  a  large  vein,  carrj'ing  in  places  at  least  good  values. 

EXTENSION   OF   CORRELATION   TO   THE    WANDERING    BOY   AND   GOLD    HILL. 

It  seems  to  the  writer,  moreover,  that  the  andesite  in  the  Fraction  No.  1  i> 
identical  with  that  in  the  Wandering  Boy,  which  is  more  nearly  the  Mizpah  Hill 
type  of  earlier  andesite.  On  following  the  chain  still  farther,  the  andesite  in  the 
Fraction  and  that  in  the  Wandering  Boy  seem  to  be  identical  and  are  probably 
in  the  same  fault  block  as  the  Gold  Hill  andesite.  The  rock  of  Gold  Hill  has 
certain  peculiarities  which  at  one  time  caused  the  writer  to  study  for  some  time 
the  question  carefully  as  to  whether  or  not  it  belonged  to  the  earlier  or  later 


WEST    END    ANDESITES.  187 

andesite,  thus  bringing  up  again  the  question  of  the  exact  age,  which  has  just 
been  raised  with  respect  to  what  is  probably  the  corresponding  rock  in  the 
West  End.  It  was  found,  however,  that  the  peculiarities  which  suggested  the 
correlation  of  the  Gold  Hill  andesite  with  the  later  andesite,  namely,  the  fre- 
quently large-sized  feldspars  and  the  presence  of  biotite,  could  be  paralleled  in 
specimens  found  in  Mizpah  Hill,  even  in  the  workings  of  the  Mizpah  mine,  and 
again  in  the  Montana  Tonopah,  where  there  was  no  question  as  to  the  andesite 
being  other  than  the  earlier  andesite. 

Moreover,  in  Gold  Hill  this  andesite  incloses  veins  having  all  the  character- 
istics of  the  veins  found  in  Mizpah  Hill,  such  as  have  not  been  found  in  the 
undoubted  later  andesite.  Therefore  the  evidence  decidedly  favors  the  conclusion 
that  the  Gold  Hill  rock  is  the  earlier  andesite.  If  it  is  true,  as  has  been  con- 
cluded, that  the  veins  of  the  Wandering  Boy  and  the  Fraction  were  originally  a 
part  of  the  Valley  View  system  and  that  they  were  displaced  by  faulting,  the 
evidence  grows  still  stronger. 

THE    WEST   END   ANDESITE    PROBABLY    EARLIER    ANDESITE. 
l.(  • 

The  writer  is  forced  to  the  conclusion  that  the  andesite  exposed  on  the  200- 
foot  level  of  the  West  End  belongs  to  the  earlier  andesite. 

CONTACT    BETWEEN    EARLIER   AND   LATER   ANDESITES. 

PLACE   AND   CHARACTER  OF  CONTACT. 

The  conclusion  that  the  rock  on  the  220-foot  level  is  the  earlier  andesite 
having  been  reached,  the  question  comes  up  as  to  the  line  of  demarcation  between 
the  earlier  andesite  below  and  the  later  andesite  above.  Since  the  West  End 
fault  probably  dips  southwestward  and  is  normal,  the  shaft,  after  passing  through 
the  fault  and  leaving  the  rhyolite,  is  in  the  block  lying  northeast  of  the  fault, 
which  may  be  called  the  Midway  block.  This  block  is  characterized  at  the  sui'- 
face  everywhere  by  undoubted  later  andesite.  It  is,  then,  likely  that  the  contact 
between  the  later  andesite  and  the  earlier  andesite  occurs  in  the  West  End  shaft 
somewhere  above  196  feet,  and  from  considerations  given  it  may  be  assumed, 
temporarily  at  least,  that  it  lies  between  116  and  196  feet  (see  p.  185). 

This  assumption  is  rendered  somewhat  doubtful  by  the  fact  that  no  contact 
was  observed,  but,  on  the  other  hand,  the  rock  is  thoroughly  decomposed  and 
much  disturbed  by  faulting,  so  that  the  presence  of  a  contact  would  be  obscured. 

NATURE    OF   SIMILAR    CONTACTS    ELSEWHERE. 

At  another  point  where  the  writer  has  seen  the  contact  between  the  over- 
lying later  andesite  and  underlying  earlier  andesite,  in  the  same  fault  block,  at 
the  Tonopah  Extension,  the  contact  is  by  no  means  striking,  and  could  not  be 


188  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 

distinguished  if  the  rock  was  much  decomposed  or  faulted.  In  the  Tonopah 
Extension  this  contact  is  at  a  depth  of  about  18-t  feet  from  the  surface  and  is 
nearly  flat. 

Similarly  in  the  Midway  mine,  which  is  very  likely  in  the  same  block,  the 
contact  between  the  overlying  later  andesite  and  the  underlying  earlier  andesite 
could  not  be  definitely  located,  probably  on  account  of  the  great  decomposition 
of  the  rocks  at  this  place. 

The  earlier  andesite  in  the  Tonopah  Extension,  moreover,  partakes  very 
largely  of  the  characteristics  of  the  Fraction  andesite,  and  in  many  cases  resem- 
bles somewhat  the  later  andesite,  but  is  elsewhere  quite  typical,  and  contains 
strong  veins,  which  show  in  places  high  values  and  evidently  belong  to  the  earlier 
andesite  series  of  veins,  so  that  there  can  be  no  doubt  as  to  its  identity. 

TONOPAH    RHYOLITE-DACITE. 

Andesite  similar  to  that  on  the  200-foot  level  continues  down  in  the  shaft  to 
390  feet,  at  which  point  a  slight  breccia  is  encountered,  striking  N.  70C  E.  and 
dipping  northwest  at  an  angle  of  45°.  Below  this  a  quartz  vein  is  encountered, 
with  highly  silicified  Tonopah  rlvyolite-dacite  as  its  walls. 

On  the  500-foot  level  drifts  run  north  and  south  about  300  feet  in  all.  There  are 
also  crosscuts.  The  whole  is  entirely  in  rhyolite-dacite.  The  rock  is  intensely 
silicitied.  being  in  places  nearly  solid  quartz,  and  contains  pyrite  throughout,  but 
there  are  no  definite  veins.  This  quartz  is  barren,  although  assays  of  $51  or  $2  have 
been  obtained  in  places.  The  rock  is  characteristically  intensely  fractured,  and  in 
places  contains  open  fissures  running  in  a  direction  somewhat  east  of  north.  These 
fissures  when  cut  contain  the  heavy  gas  elsewhere  referred  to  as  being  probably 
carbonic  acid  (see  p.  !)4).  The  probable  explanation  is  that  the  gas  was  formed  in 
the  overlying  soft  andesite  by  the  reaction  of  acids  upon  the  contained  calcite, 
and  by  its  weight  sank  into  the  fissures  in  the  underlying  rigid  rhyolite-dacite 
and  there  accumulated. 

EARLIER   ANDESITE   AT   BOTTOM    OF   SHAFT. 

At  a  depth  of  about  680  feet  in  the  shaft  there  is  a  sharp  contact  between  the 
rhyolite-dacite  above  and  a  fine-grained  green  variety  of  earlier  andesite  below. 
This  contact  is  said  to  dip  east  at  an  angle  of  about  40°.  The  bottom  of  the  shaft 
is  at  a  depth  of  780  feet,  and  specimens  taken  from  here  and  from  below  the  contact 
show  earlier  andesite  of  a  type  very  much  like  that  on  the  700-foot  level  of  the 
Siobert  shaft. 


WORKINGS    PARTLY    IN    EARLIER    ANDESITE.  189 

MACNAMARA  WORKINGS. 
LATER   AXDE8ITK   AT   SURFACE. 

The  MacNamara  shaft  is  situated  a  short  distance  northwest  of  the  West 
End,  and  probably  in  the  same  fault  block.  The  geology  partakes  of  the  same 
perplexing  character  as  that  described  in  the  West  End  (see  p.  184).  The  shaft 
was  first  sunk  to  a  depth  of  200  feet,  from  which  point  drifts  were  run  50 
feet  to  the  north  and  about  300  feet  to  the  south.  The  rock  in  which  the  shaft 
started  and  which  outcrops  in  the  vicinity  is  undoubted  later  andesite,  such  as 
covers  the  whole  surface  of  this  fault  block. 

CHARACTER  OF   ANDESITE   ON    200-FOOT   LEVEL. 

The  rock  encountered  on  the  200-foot  level  differs  in  character  very  slightly 
from  that  at  the  surface,  except  that  the  latter  has  the  purplish  color  due  to 
partial  oxidation,  while  the  former  has  a  green  color  characteristic  of  andesite, 
containing  a  large  proportion  of  chlorite  as  a  result  of  subterranean  alteration 
processes.  Also  the  andesite  at  the  surface  is  decidedly  fresher  than  that  on  the 
200-foot  level,  where  it  is  always  highly  altered. 

CORRELATION    OF   MACNAMARA    AXD   WEST    END   AXDES1TES. 

There  would,  however,  be  hardly  sufficient  reason  for  dividing  the  upper 
and  the  lower  andesite  were  it  not  that  study  and  comparison  make  it  seem  clear 
that  the  rock  on  the  200-foot  level  is  practical!}1  identical  in  characteristics  with 
that  on  the  220-foot  level  of  the  West  End,  which  the  writer,  for  reasons 
previously  given,  is  obliged  to  believe  to  be  a  phase  of  the  earlier  andesite  rather 
than  of  the  later  andesite. 

The  MacNamara  rock  can  be  matched  almost  exactly  with  specimens  of  the 
West  End  rock.  When  studied  under  the  microscope  it  is  found  to  be  altered 
largely  to  chlorite  and  calcite,  with  pyrite,  quartz,  siderite.  and  sericite.  If  it  is 
the  earlier  andesite,  therefore,  it  belongs  to  that  phase  which  has  altered  to  calcite 
and  chlorite  rather  than  to  that  which  has  altered  to  quartz  and  muscovite.  such 
as  the  phase  found  on  the  700-foot  level  of  the  Siebert  shaft  and  below,  which  is 
believed  by  the  writer  to  have  been  formed  usually  at  some  distance  from  the 
mineral-bearing  veins  rather  than  in  their  immediate  proximity. 

This  rock  contains  calcite  blotches  and  veinlets,  and  occasional  stringers  of 
mixed  quartz  and  calcite,  one  of  which,  it  is  claimed,  afforded  assays  showing  a 
value  of  $2. 


190  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 

CONTACT   OF    EARLIER   AND    LATER    ANDESITES. 

Since  it  therefore  seems  necessary  to  distinguish  between  the  andesite  near 
the  surface  and  that  on  the  200-foot  level,  the  question  as  to  the  line  of  contact 
comes  up.  According  to  the  conclusions  arrived  at  this  must  exist,  although  it 
is  very  difficult  to  distinguish  it.  From  a  study  of  the  rock  in  the  shaft  and 
from  specimens  taken  there,  the  approximate  boundary  line  has  been  placed  at  a 
point  125  feet  from  the  surface,  where  a  change  of  formation  was  recognized  by 
the  miners  in  sinking.  This  also  would  correspond  fairly  well  with  the  conclusions 
in  respect  to  the  West  End,  where  the  contact  was  placed  between  ll(i  and  196  feet 
from  the  surface,  and  with  that  in  the  Tonopah  Extension,  where  it  has  been 
placed  at  184  feet  from  the  surface. 

TONOPAH    RHYOLITE-UACITE    AND   INCLUDED   VEINS. 

At  a  depth  of  285  feet  a  light-colored  altered  rock  (Tonopah  rhyolite-dacite) 
was  struck  beneath  the  green  andesite.  At  the  contact,  which  strikes  east  and 
west  and  dips  north  at  an  angle  of  45°,  was  a  heavy  zone  of  ground-up  material. 
The  rock  immediately  beneath  this  breccia  contained  a  barren  quartz  ledge,  about 
16  feet  thick,  striking  and  clipping  nearly  parallel  with  the  contact,  while  beneath 
this  were  numerous  quartz  stringers.  This  rhyolite-dacite  proves  on  examination 
to  be  entirely  altered,  chiefly  to  quartz  and  sericite.  with  probable  kaolin.  Original 
phenocrysts  consisted  of  small  and  rather  sparse  crystals  of  feldspar  and  biotite. 
and  in  one  case  a  small  crystal  of  quartz.  This  rock  is  the  same  as  that  which  was 
found  in  the  lower  part  of  the  neighboring  West  End  shaft. 

Besides  the  level  at  a  depth  of  200  feet,  already  described,  there  are  levels  at 
depths  of  855  and  500  feet.  At  the  355-foot  level  a  drift  runs  a  short  distance 
northwest  of  the  shaft  and  encounters  a  heavy  but  irregular  quartz  vein,  having  a 
general  east- north  east  strike,  and  a  moderate  northwest  dip.  This  vein,  as  shown 
in  the  section  (tig.  71),  lies  very  nearly  parallel  with  the  upper  contact  of  the 
rhyolite-dacite  and  the  earlier  andesite.  a  short  distance  above.  It  consists  of 
white  quartz,  and  also  of  gray  and  black  jiispery  quartz.  It  is  in  general  barren, 
but  in  places  small  assays  have  been  obtained.  It  is  cut  by  several  faults,  of 
which  the  chief  ones  strike  northeast  and  dip  steeply  southeast.  The  effect  of 
these  seems  to  be  in  general  to  cause  a  movement  as  if  the  vein  had  been  thrown 
down  on  the  southeast  side.  One  of  these  faults,  marked  by  a  heavy  drag  of 
quartz  and  rock  breccia,  has  been  followed  by  a  drift  for  a  few  hundred  feet  to 
the  southwest.  Near  the  end  of  the  drift  the  fault  splits  and  both  forks  have 
been  followed  a  short  distance.  In  one  of  these  branch  drifts  a  small  bunch  of 
ore,  carrying  very  good  values,  is  reported  to  have  been  found.  Specimens  of 
this  ore,  shown  to  the  writer,  were  composed  of  white  quartz  containing  argen- 


VEINS    AT    CONTACT    OF    ODDIE    BHYOLITE. 


191 


tite,  ruby  silver,  polybasite,  or  stephanite.  This  occurrence  of  bunches  of -high- 
grade  ore,  probably  belonging  to  veins  of  the  later  rhyolite-dai-ite  period,  is 
similar,  to  that  of  ore  in  veins  of  the  same  period  in  the  Desert  Queen. 

At  the  500-foot  level,  which  is  at  the  bottom  of  the  shaft,  the  formation  is 
all  rhyolite-dacite.  It  has  been  explored  for  a  short  distance  north,  south,  and 
west  by  drifts.  No  bodies  of  quartz  of  any  importance  were  found,  although  a 
drift  along  a  northeast  fault  plane  shows  a  breccia  partly  cemented  by  jasperv 
quartz. 

SE  .,_..,  .     „.  NW 

Tonopah  Extension  shaft 


MacNamara  shaft 


Fin.  71. — Vertical  section  through  Mm-Samarn  and  Tonopah  Extension  shrfts. 

EXPLORATIONS    ON   VEINS    AT    THE    CONTACT    OF    THE    ODDIE 

RIIYOL.ITE. 

WINGFIELD    TUNNEL. 

The  Wingtield  tunnel  is  situated  on  the  southwest  slope  of  Ararat  Mountain. 
It  starts  in  later  andesite  near  the  contact  of  this  rock  with  the  Oddie  rhyolite, 
which  forms  the  summit  of  the  mountain,  and  passes  from  the  later  andesite 


192  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 

across  the  contact  into  the  rhyolite.  It  is  160  feet  long  and  runs  N.  60C  E. 
At  the  breast  of  the  tunnel  the  rock  is  very  much  shattered  Oddie  rhyolite 
containing1  openings  filled  with  brown  iron-lime  carbonate  and  white  calcite. 
From  this  point  to  the  contact  with  the  later  andesite  the  rock  is  mostly  a  dense 
rhyolite  breccia  of  volcanic  origin,  the  fragments  being  of  very  large  size. 
Strong  open  fractures  striking  N.  25°  W.  and  dipping  east  at  an  angle  of  60° 
are  lined  with  white  and  brown  carbonates,  oxidized  in  places  to  iron  oxide 
and  manganese  oxide.  Throughout  the  breccia,  tilling  all  the  interspaces,  are 
veinlets,  filled  chiefly  with  ferriferous  carbonate  and  to  a  less  degree  with  calcite 
and  chalcedony.  Veins  of  smooth  brown  or  bluish  jasper,  indicating  silicification 
of  the  rhyolite,  have  the  same  course  and  the  whole  breccia  is  largely  silicitied. 
Some  of  this  material  is  claimed  to  run  $8  or  $9  to  the  ton,  the  values  being  all 
in  gold. 

The  contact  of  the  andesite  with  the  rhyolite  is  70  feet  from  the  mouth  of 
the  tunnel,  and  strikes  N.  35°  W.  and  dips  east  at  an  angle  of  50°.  The  rhyolite 
is  plainly  intrusive.  The  brecciation,  fracturing,  and  silicitication  of  the  rhyolite 
increase  in  measure  as  the  contact  is  approached.  Near  the  mouth  of  the  tunnel 
two  dikes  of  rhyolite  breccia,  one  6  inches  thick  and  one  3  feet  thick,  lie  in  the 
andesite.  These  are  in  general  parallel  to  the  main  contact,  but  dip  50°  in  an 
opposite  direction.  The  fracturing  and  brecciation  are  confined  to  the  rhyolite, 
and  are  not  notable  in  the  later  andesite,  which,  however,  is  highly  decomposed 
and  crumbling,  while  the  rhyolite  is  hard. 

The  evident  interpretation  of  these  phenomena  is  that  this  rhyolite  column 
was  intruded  into  the  andesite  and  that  the  upward  movement  continued  after 
the  beginning  of  cooling.  The  result  of  this  upward  impulse  was  that  the  cooler 
rhyolite  for  a  zone  of  nearly  100  feet  thick  near  the  contact  was  intensely 
brecciated  while  in  n  semisolid  state.  The  upward  pressure  continued  even  after 
further  cooling,  causing  open  fractures,  mostly  parallel  to  the  contact,  but 
sometimes  cutting  across  the  rhyolite,  as  has  been  described  elsewhere  (p.  101). 
Along  these  open  fractures  ascending  hot  waters,  whose  advent  followed  the 
eruption,  deposited  iron  and  lime  carbonates,  silica,  some  manganese,  and  probably 
some  gold. 

BOSTON  TONOPAH  SHAFT. 

The  Boston  Tonopah  shaft,  lies  200  or  300  feet  south  of  the  Wingtield  tunnel, 
farther  down  the  slope.  At  the  time  of  the  writer's  visit  it  was  300  feet  deep,  230 
feet  in  the  later  andesite  and  the  last  70  feet  in  white  rhyolite  like  that  constituting 
the  central  plug.  The  contact  between  the  andesite  and  the  rhyolite  in  the  shaft, 
according  to  Mr.  McCambridge,  the  superintendent,  pitches  northwest. 


SHAFTS    AT    CONTACT    OF    ODDIE    BHYOLITE. 


MIRIAM   SHAFT. 


193 


On  the  Miriam  claim  a  shaft  about  -iO  feet  deep  had  been  sunk  at  the  time  of 
the  writer's  visit..  This  shaft  lies  about  1,200  feet  southeast  of  the  Belle  of  Tonopah 
and  is  at  the  contact  of  rhyolite  and  later  andesite.  It  cuts  at  the  top  30  feet  of 
brown  decomposed  later  andesite  and  below  this  10  feet  of  white  rhyolite,  which 
is  intrusive  into  the  andesite.  The  rhyolite  is  typical  and  shows  abundant  quartz 
and  orthoclase  phenocrysts  with  brown  glassy  groundmass.  From  some  streaks 
along  this  contact  assays  in  gold  were  obtained,  with  no  silver. 

DESERT  QUEEN  SHAFT. 

At  a  depth  of  920  feet  the  Desert  Queen  shaft  passed  into  the  Oddie  rhyolite, 
the  contact  being  flat.     Twelve  feet  below  this  there  was  encountered  a  nearly  flat 
quartz  vein,  which  is  parallel  with  the  rhyolite  contact  and  consists  of  white  or  red 
NE.  sw. 

Surface 


Scale 

5 


10  feet 


FIG.  72.— Vertical  sketch  section  of  shallow  trench  just  north  of  Belmont  shaft,  showing  contact  of  the  Oddie  rhyolite 
intrusion  with  the  later  andesite.    a=Oddie  rhyolite;  6=later  andesite. 

quartz  carrying  some  pyrite.  This  quartz  was  7  feet  thick  and  had  as  a  foot  wall 
the  same  body  of  rhyolite.  The  highest  of  several  assays  made  showed  0.08  ounce 
gold  and  2.12  ounces  silver,  with  a  little  galena  and  traces  of  arsenic  and  copper. 
As  this  practically  barren  vein  is  within  the  Oddie  rhyolite,  it  must  be  of  later  origin 
than  the  rich  veins  in  the  earlier  andesite.0 

SHAFTS  AT  THE  UNMINEHALJZED  CONTACT  OF  THE  OUDIE  RHYOLITE. 

BELMONT  SHAFT. 

The  Belmont  shaft  (distinct  from  the  Desert  Queen  shaft,  which  is  also  on 
the  Belmont  property)  is  situated  on  the  north  side  of  Rushton  Hill.  At  the 
time  of  the  writer's  last  visit,  in  July,  1903,  the  shaft  was  340  feet  deep,  all  in 

a  For  the  description  of  the  geology  of  the  rest  of  the  Desert  Queen  shaft,  see  p.  125. 
16843— No.  42—05 13 


194  GEOLOGY    OF   TONOPAH   MINING   DISTRICT,   NEVADA. 

hard  white  rhyolite.  It  is  located  about  200  feet  south  of  the  contact  of  the 
rhvolite  with  the  later  andesite.  This  contact  is  exposed  in  a  short  trench  and 
in  a  pit  about  8  feet  deep,  and  the  rhyolite  is  seen  to  be  intrusive  into  the 
andesite,  with  an  approximately  perpendicular  contact  (fig.  72).  This,  together 
with  the  depth  of  the  Belmont  shaft,  indicates  that  it  is  being  sunk  in  the 
Rushton  Hill  neck  (which  is  a  part  of  and  is  connected  with  the  Mount  Oddie 
neck)  at  a  point  where  the  contact  is  very  steep. 

RESCUE  SHAFT. 

The  Rescue  shaft  is  located  south  of  Mount  Oddie,  about  one-fourth  of 
a  mile  southeast  of  the  Desert  Queen  shaft.  It  is  near  the  contact  of  the  white 
rhyolite  which  makes  up  Mount  Oddie  and  Rushton  Hill  with  the  later  andesite. 
The  contact  is  exposed  at  the  surface,  about  120  feet  north  of  the  shaft,  and 
here  has  a  general  east-west  strike  and  a  southerly  dip  of  from  45°  to  60°.  The 
contact  is  intrusive  and  there  is  some  slight  brecciation  of  the  intrusive  rock  in 
the  bends  of  the  lobes  which  jut  into  the  intruded  rock,  showing  squeezing  of  the 
upflowing  lava  at  these  points. 

The  shaft,  which  starts  in  the  later  andesite,  cuts  the  same  contact  as  has 
been  described  in  outcrop,  at  a  depth  of  60  feet.  This  contact  pitches  in  the 
shaft  about  45°  to  the  south.  From  this  point  to  a  depth  of  410  feet,  which 
the  shaft  had  attained  at  the  time  of  the  writer's  visit  in  November,  1904,  the 
rock  was  entirely  white  rhyolite  of  the  Oddie  Mountain  type.  From  this  it 
will  be  seen  that  the  shaft  is  being  sunk  in  the  intrusive  rhyolite  neck. 

Water  has  been  encountered  in  this  shaft  (see  p.  105). 

EXPLORATIONS     ON     VEINS     AT     THE     CONTACT     OF     THE    TONOPAH 

RHYOLJTE-DACITE. 

MIZPAH     EXTENSION     SHAFT. 
LATER   ANDESITE   AT   TOP  OF   SHAFT. 

The  Mizpah  Extension  shaft  is  sunk  in  the  hollow  between  the  two  white 
rhyolite  intrusions  of  Mount  Oddie  and  Ararat  Mountain.  The  later  andesite 
outcrops  between  these  two  intrusions,  and  on  account  of  its  relative  softness 
has  been  worn  away  to  form  the  depression  separating  the  two  hills.  The 
shaft  was  started  in  this  later  andesite,  and  continued  in  it  down  to  a  depth  of 
about  200  feet.  The  rock  is  of  a  general  purplish  color,  with  large  white  feldspars 
and  biotite  phenocrysts.  At  a  depth  of  about  200  feet,  however,  a  variety  of 
this  is  tine  grained,  black,  almost  basaltic  looking,  and  is  fresher  than  the  rest 
of  the  rock,  which  is  sometimes  considerably  decomposed. 


MIZPAH    EXTENSION    SHAFT.  195 

RHTOLITE   AND    RHYOLITE-DACITE   IX    SHAFT. 

At  a  depth  of  300  feet  the  andesite  is  in  contact  with  an  underlying  typical 
white  rhyolite,  like  that  of  Mount  Oddie.  This  contact  strikes  about  N.  60°  W. 
and  dips  northeast  at  from  20°  to  25°.  Both  andesite  and  rhyolite  have  been 
softened  near  the  contact  by  circulating  waters,  so  that  their  contact  phenomena 
are  not  observable.  At  a  depth  of  about  430  feet  in  the  shaft  the  rhyolite  comes 
in  contact  with  a  rock  referred  to  the  glassy  Tonopah  rhyolite-dacite.  This 
contact  strikes  N.  30°  W.  and  dips  northeast  at  an  angle  of  40°,  and  is  marked 
by  about  14  feet  of  wet  clay,  decomposed  and  containing  bowlders.  Some  water 
runs  on  top  of  this  clay  zone. 

VEINS   AT   CONTACT   OF   TONOPAH   RHYOLITE-DACITE. 

Immediately  below  the  contact,  but  in  the  Tonopah  rhyolite-dacite,  a  large 
quartz  vein  comes  in.  This  vein  is  several  feet  thick,  and  has  the  same  attitude 
as  the  contact.  Indeed,  it  appears  to  follow  the  contact,  although  it  lies  in  the 
rhyolite-dacite.  At  a  depth  of  500  feet  a  drift  was  run  for  the  puipose  of 
developing  this  vein.  The  lower  contact  of  the  vein  in  the  shaft  (at  465  feet) 
has  a  strike  of  N.  70°  W.  and  a  northeast  dip  of  45°,  but  it  is  much  natter 
between  this  point  and  the  point  at  which  it  was  cut  in  the  drift,  where  it 
has,  however,  the  same  general  strike.  In  this  drift,  which  runs  in  an  irregular 
course  for  upward  of  150  feet,  the  vein  is  displaced,  not  far  from  the  shaft,  by  a 
vertical  fault  having  a  strike  of  N.  45°  E.  The  displacement  of  this  fault  is  not 
known,  as  the  vein  was  not  looked  for  on  the  southeast  side.  On  the  northeast  side 
it  was  drifted  on  for  some  little  distance,  and  continued  strong.  This  vein  is  an 
ordinary  quartz  vein  which  is  not  very  dissimilar  in  appearance  from  the  average 
vein  in  the  earlier  andesite,  but  which  contains  a  notably  large  amount  of  pvrite. 
It  has  locally  a  banded  structure,  which  is  probably  due  chiefly  to  replacement. 
Nevertheless,  the  vein  is  ordinarily  nearly  barren,  the  highest  assay  obtained 
having  been  about  $12.  The  proportion  of  values  differed  from  the  ordinary 
Tonopah  vein  in  that  they  were  about  75  per  cent  gold  and  25  per  cent  silver. 

At  a  depth  of  505  feet  in  the  shaft  another  quartz  vein  was  encountered, 
several  feet  thick,  with  characteristics  like  the  one  above.  This  vein  has  a  strike 
of  N.  55°  W.  and  a  dip  of  55°  to  the  northeast.  A  specimen  of  the  wall  rock 
taken  immediately  below  this  vein  proved  to  be  andesite,  probably  later  andesite. 
Therefore  this  vein  appears  to  occur  on  the  under  contact  of  the  Tonopah  rhyolite- 
dacite  with  the  later  andesite,  while  the  first-mentioned  vein  occurs  on  the  upper 
contact  of  the  same  rhyolite-dacite  body. 

This  rhyolite-dacite  is  similar  to  that  which  outcrops  to  the  north  of  Ararat 
Mountain,  is  like  that  discovered  in  depth  in  the  Desert  Queen  and  Siebert  shafts. 


196  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,   NEVADA. 

and  is  very  similar  to  some  of  the  rock  at  Gold  Mountain  (4  miles  south  of  Tonopah) 
in  the  Tonopah  Union  shaft.  It  has  a  pyroclastic  structure,  with  occasional  pheno- 
crysts  of  quartz  and  more  common  crystals  of  feldspar,  chiefly  orthoclase,  which 
are  largely  altered  to  quartz  and  muscovite.  Small  biotite  crystals  are  also  altered 
to  white  mica  by  bleaching.  The  groundmass  is  a  microfelsitic  devitrified  glass. 
Some  secondary  adularia  was  observed. 

From  the  lower  contact  of  this  rhyolite-dacite  body  the  shaft  passes  through 
later  andesite  again  to  a  depth  of  about  620  feet,  making  the  thickness  of  this 
body  of  andesite  traversed  somewhat  over  100  feet.  At  this  point  again  there 
is  a  contact  between  the  later  andesite  above  and  Tonopah  rhyolite-dacite  below 
similar  to  that  just  described.  A  short  distance  from  the  contact  a  vein  of  quartz 
2  feet  thick,  containing  pyrite  and  otherwise  having  the  same  characteristics  of 
the  upper  veins,  was  encountered  in  the  rhyolite-dacite.  This  also  seems  to  be 
very  nearly  a  contact  vein.  The  bottom  of  the  shaft,  at  a  depth  of  800  feet, 
is  still  in  the  same  rhyolite-dacite. 

From  the  bottom  of  the  shaft  a  drift  was  run  due  east  525  feet  since  the 
visit  of  the  writer.  A  specimen  of  the  rock  sent  to  the  writer  from  the  end  of 
the  drift  is  rhyolite-dacite,  like  that  at  the  bottom  of  the  shaft,  but  a  specimen 
taken  from  an  intermediate  point  in  the  drift  is  later  andesite.  Mr.  C.  E.  Knox, 
the  president  of  the  company  which  has  conducted  these  explorations,  reports  that 
the  veins  cut  in  the  shaft  were  cut  again  in  this  drift  in  regular  order.  It  is 
probable,  therefore,  that  the  alternating  bands  of  rock,  striking  northwest  and 
dipping  southeast,  were  encountered  in  the  drift  also,  with  the  exception  perhaps 
of  the  white  rhyolite,  which  has  not  been  reported  as  occurring  in  the  drift.  It  is 
interesting  to  note  that  the  end  of  the  drift  has  been  carried  somewhat  past  the 
surface  contact  of  the  rhyolite-dacite  with  the  later  andesite  perpendicularly  above 
on  the  slopes  of  Ararat  Mountain. 

CORRELATION    OF   THE    RHYOLITIC   ROCKS   IN   THE    SHAFT. 

As  before  stated,  the  contact  phenomena  were  not  observable  in  the  mine  on 
account  of  alteration  by  circulating  waters,  but  from  what  has  been  observed  at 
other  points  in  the  district  it  may  be  believed  that  here,  too,  the  andesite'  is  the 
older  of  the  rocks  exposed;  that  it  has  been  cut  by  the  Tonopah  rhyolite-dacite, 
and  that  the  white  rhyolite  was  the  last  of  all  and  is  also  of  an  intrusive  nature. 
The  form  of  the  different  igneous  bodies  underground  must  be  very  complex,  and 
it  is  difficult  or  impossible  to  even  outline  the  connections  between  the  similar 
lavas.  It  seems  likely,  however,  that  the  white  rhyolite  is  connected  with  that 
of  Mount  Oddie,  and  the  Tonopah  rhyolite-dacite  with  that  around  Ararat 
Mountain. 


KING    TONOFAH    SHAFT. 
AGE   OF   THE   VEINS. 

The  veins  clearly  belong  to  a  period  subsequent  to  the  formation  of  the  veins 
in  the  earlier  andesite,  as  shown  by  their  having  the  Tonopah  rhyolite-dacite  for 
a  wall  rock.  The  relatively  high  content  in  gold  as  compared  with  silver  seems 
to  be  very  common  in  these  post-andesitic  veins  connected  with  the  dacite- 
rhyolites. 

KING  TONOPAH  SHAFT. 
GEOLOGICAL   SITUATION. 

The  King  Tonopah  shaft  lies  at  the  contact  of  the  Tonopah  rhyolite-dacite  with 
the  later  andesite.  At  many  points  along  the  irregular  contact  of  these  two  rocks 
phenomena  were  observed  indicating  that  the  rhyolite-dacite  is  intrusive  into  the 
andesite.  The  rhyolite-dacite  sends  out  intrusive  irregular  projections  into  the 
andesite,  and  isolated  dikes  or  necks  appear  in  the  andesite  some  little  distance  away 
from  the  contact. 

The  shaft  starts  in  the  later  andesite,  and  at  a  depth  of  38  feet  passes  into 
silicified  rhyolite-dacite.  The  total  depth  of  the  shaft  is  300  feet,  and  from  the 
bottom  a  drift  was  run  to  the  north  arid,  at  the  time  of  the  writer's  visit,  extended 
48  feet  from  the  shaft. 

VEIN   MATERIALS. 

At  a  depth  of  226  feet  a  zone  of  silicified  rhyolite-dacite  with  quartz  stringers 
was  cut  in  the  shaft.  It  is  several  feet  in  thickness,  but  was  practically  barren  of 
values,  the  highest  assay  reported  being  only  about  $2.  Some  of  this  vein  material 
contains,  besides  quartz,  abundant  adularia,  as  is  shown  by  microscopic  study. 
There  is  also  some  finely  striated  feldspar,  which  may  be  albite.  Some  of  the 
adularia  shows  the  characteristic  rhombic  cross  sections,  and  many  of  these  crystals 
are  entirely  inclosed  in  quartz. 

NATURE   OF   ROCK   INCLOSING   VEIN    MATERIALS. 

The  rock  in  which  this  material  lies  and  in  which  the  entire  shaft  and  drift  has 
been  driven  below  a  depth  of  38  feet  is  the  Tonopah  rhyolite-dacite.  It  is  a  glassy 
lava  made  up  for  the  most  part  of  a  glassy  grounduiass,  usually  more  or  less  devitri- 
fied  and  altered  to  quartz,  kaolin,  and  sericite  aggregates.  In  some  specimens 
abundant  fine  adularia  of  secondary  origin  has  been  found  in  the  groundmass. 
Scattered  small  crystals  of  feldspar  usually  occur,  but  they  are  mostly  nearly  or  quite 
altered  to  sericite  and  sometimes  to  adularia.  The  blunt  form  of  some  of  these 
crystals  shows  probable  original  orthoclase,  while  some  are  more  elongated,  suggest- 
ing a  more  basic  species. 


198  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 

CORRELATION    OF   VEINS   WITH    OTHER   OCCURRENCES. 

The  formation  of  pyritiferous  quartz  veins  in  this  rock  is  therefore  a  contact 
phenomenon  near  the  edge  of  the  intrusion,  for  the  glassy  rhyolite-dacite  outcropping 
away  from  the  contact  is  usually  quite  fresh  and  unsilicified.  This  idea  gains  cor- 
roboration  from  the  fact  that  near  by,  at  other  points  on  the  contact,  namely,  in  the 
vicinity  of  the  Belle  of  Tonopah  shafts  and  elsewhere,  silicification  and  the  formation 
of  veins  has  occurred.  This  vein  is,  then,  of  the  same  class  as  the  veins  at  the  contact 
of  the  glassy  Tonopah  rhyolite-dacite  in  the  Mizpah  Extension  and  other  shafts. 

BELLE  OF  TONOPAH  SHAFT. 
GEOLOGICAL   CONDITIONS. 

The  Belle  of  Tonopah  is  situated  in  the  northern  corner  of  the  area  mapped,  about 
1,600  feet  west  of  the  King  Tonopah,  on  the  irregular  contact  of  the  glassy  Tonopah 
rhyolite-dacite  and  the  later  andesite.  At  a  number  of  places  along  this  contact  the 
phenomena  show  that  the  rhyolite-dacite  is  intrusive  into  the  andesite.  Just  south 
of  the  Belle  of  Tonopah  shaft  a  number  of  rhyolite-dacite  dikes  occur  in  the  later 
andesite  near  the  contact.  These  are  considerably  decomposed  and  are  accompanied 
by  small  and  nonpersistent  quartz  veins,  which  give  assays  showing  generally  small 
and  irregular  quantities  of  gold,  with  little  silver. 

The  Belle  of  Tonopah  shaft  starts  in  such  a  rhyolite-dacite  dike,  very  close  to  the 
contact,  and  passes  downward  through  20  feet  of  this  material,  when  it  enters  the 
later  andesite.  The  contact  of  the  two  rocks  strikes  west-northwest  and  dips  south- 
west at  angles  ranging  from  65°  or  70°.  The  contact  is  marked  by  a  decomposed 
zone,  and  the  later  andesite  below  is  soft  and  is  very  full  of  pyrite  which,  however, 
is  quite  barren. 

At  the  time  of  the  writer's  visit  the  shaft  was  230  feet  deep,  all  except  the 
upper  20  feet  being  in  the  later  andesite.  Since  then,  in  January,  1904,  Mr. 
A.  C.  Stock,  the  manager,  has  sent  the  writer  a  specimen  of  the  rock  from  the 
bottom  of  the  shaft  at  a  depth  of  460  feet.  This  is  later  andesite. 

The  rhyolite-dacite  in  the  upper  part  of  the  shaft  resembles  the  rock  from 
the  King  Tonopah.  It  is  highly  decomposed,  but  has  the  structure  of  a  nearly 
glassy  volcanic  rock  and  contains  many  very  small  crystals,  nearly  all  of  which 
seem  to  have  been  feldspar,  with  no  original  quartz.  Now  the  whole  rock  is 
altered  to  kaolin,  chert,  hematite,  siderite,  etc.  This  rock  unquestionably  belongs 
to  the  glassy  Tonopah  rhyolite-dacite  and  is  intrusive  into  the  later  andesite  at 
this  place. 


LITTLE    TONOPAH    SHAFT.  199 

VEINS. 

The  quartz  stringers  found  along  the  edges  of  the  rhyolite  dikes  near  the 
edge  of  the  shaft  are  stated  by  Mr.  Stock  to  run  as  high  as  $13  in  gold.  These 
consist  of  dark,  rather  dense  quartz,  carrying  a  great  deal  of  pyrite.  In  the 
shaft  also  small  stringers  have  been  found  up  to  the  time  of  the  writer's  visit, 
generally  striking  parallel  with  the  slips  but  dipping  in  the  opposite  direction, 
and  affording  assays  running  up  as  high  as  $18,  being  all  in  gold.  Mr.  Stock 
reports  that  at  a  depth  of  440  feet  a  stringer  was  cut  which  gave  an  assay  of 
$39.60  in  gold  and  $3.80  in  silver,  while  at  the  bottom  of  the  shaft  (460  feet) 
another  stringer  2  inches  thick  gave  an  assay  of  $4.14  in  gold  and  $6  in  silver, 
the  latter  being  the  first  which  showed  preponderating  silver  values,  the  other 
assays  from  the  shafts  of  the  neighborhood  showing  chiefly  gold.  This  minerali- 
zation is  therefore  comparable  with  the  low-grade  pyrite-bearing  quartz  veins, 
with  the  values  chiefly  in  gold,  which  occur  at  various  other  points  in  or  near 
the  glassy  Tonopah  rhyolite-dacite  near  its  contact.  It  is  due  to  the  action  of 
heated  waters  circulating  along  the  contact,  subsequent  to  the  intrusion  of  the 
rhyolite-dacite,  and  is  of  a  different  and  later  period  from  that  of  the  veins  in 
the  earlier  andesite. 

The  abundance  of  pyrite  in  the  altered  later  andesite  seems  to  indicate  that 
the  pyritization  here,  as  probably  in  the  case  of  similarly  altered  later  andesite 
on  Mizpah  Hill,  is  associated  with  present  water  courses.  The  pyrite,  like  that 
of  the  later  andesite  on  Mizpah  Hill,  is  barren  of  gold  and  silver  values. 

SHAFTS  AT  THE  UNMINERALIZED   CONTACT  OF  THE  TONOPAH 

RHYOLITE-DACITE. 

BUTTE  TONOPAH   SHAFT. 

The  Butte  Tonopah  shaft,  at  the  eastern  base  of  Ararat  Mountain,  was  35 
or  40  feet  deep  at  the  time  of  the  writer's  visit.  It  was  in  the  Tonopah 
rhyolite-dacite,  at  the  contact  of  this  rock  with  the  later  andesite.  This  con- 
tact, apparently  vertical,  is  plainly  and  continuous!}-  shown  about  30  feet  east  of 
the  shaft.  The  rhyolite-dacite  contains  many  inclusions  of  the  andesite,  and  is 
intrusive  into  it. 

LITTLE  TONOPAH  SHAFT. 

This  shaft  is  located  about  150  feet  from  the  edge  of  the  area  mapped  and 
about  one-half  mile  west  of  the  Golden  Anchor  shaft.  It  is  situated  at  the 
contact  of  the  glass}'  Tonopah  rhyolite-dacite  with  the  later  andesite.  The  shaft 
starts  in  the  rhyolite-dacite  and  runs  down  about  50  feet  to  the  contact  with  the 
andesite.  The  rhyolite  contains  fragments  of  the  later  andesite,  and  the  contact 


200  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

dips  west  at  angles  of  50°  or  55°.  The  shaft  was,  at  the  time  of  the  writer's  last 
visit  in  the  fall  of  1904,  about  585  feet  deep,  and  the  lower  part  was  all  in  the 
later  andesite.  At  the  surface  the  contact  seen  in  the  shaft  outcrops  50  or  60  feet 
east  of  the  shaft.  No  mineralization  was  observed. 

SHAFTS  AT  THE  CONTACT  OF  THE  BROTJGHER  DACITE. 

BIG  TONO  SHAFT. 

This  shaft  is  sunk  in  the  intrusive  dacite  neck  on  the  east  side  of  Brougher 
Mountain.  It  was  started  in  the  glassy  contact  phase  of  the  neck  at  the  very 
contact  with  the  dacite  breccia,  into  which  the  neck  is  intrusive.  The  shaft  is 
somewhat  over  300  feet  deep,  and  is  entirely  in  the  Brougher  dacite. 

For  a  depth  of  about  50  feet  the  glassy  phase  persists  in  the  shaft,  below 
which  is  the  ordinary  porphyritic  phase.  This  indicates  that  with  depth  the  shaft 
departs  from  the  contact,  which  at  this  point  must  pitch  away  from  the  mountain. 

MOLLY  SHAFT. 

The  Molly  shaft  is  situated  at  the  west  end  of  Golden  Mountain.  It  was 
sunk  in  the  summer  of  1903  and  was  468  feet  deep  when  work  was  stopped.  A 
rough  estimate  of  the  section  passed  through,  made  by  climbing  down  the  some- 
what tightly  lagged  shaft,  was  that  the  Brougher  dacite  occupied  the  upper  two- 
thirds,  and  the  Fraction  dacite  breccia  most  of  the  lower  third,  with  25  feet  of 
later  andesite  at  the  bottom.  There  seems  to  have  been  some  Tonopah  rh3'olite- 
dacite  sheets  in  the  Fraction  dacite  breccia.  No  water  was  encountered. 

As  shown  on  the  map,  the  shaft  lies  about  250  feet  east  of  the  nearest  point 
of  contact  of  the  Golden  Mountain  intrusive  dacite  neck  with  the  older  rocks. 
This  contact,  therefore,  has  here  a  pitch  of  about  45°  toward  the  mountain,  a 
fact  which  is  also  indicated  by  the  inward  pitch  of  the  outcropping  contact  and 
by  the  flow  structure  in  the  dacite  at  the  contact  for  some  distance  to  the  north 
and  east.  The  shaft  has  thus  passed  downward  out  of  the  dacite  neck  into  the 
older  formations. 

SHAFTS  WHOLLY  OK  CHIEFLY  IN  DACITIC  TUFFS. 

NEW  YORK  TONOPAH   SHAFT. 

The  New  York  Tonopah  lies  between  Butler  and  Brougher  mountains  and 
when  last  visited  by  the  writer  the  workings  consisted  of  only  a  shaft  745  feet 
deep.  At  the  point  where  the  shaft  was  sunk  the  surface  consists  chiefly  of 
brecciated  lavas  and  tuffs  which  have  been  referred  to  the  glass}'  Tonopah 
rhyolite-dacite.  However,  the  rocks  belonging  to  this  formation,  when  they  are 
chiefly  fragmental,  as  they  are  here,  are  often  not  easily  distinguishable,  or 


FRACTION    EXTENSION    SHAFT.  201 

perhaps  not  at  all,  from  the  tuffs  of  the  Fraction  dacite  breccia,  which  iu  general 
is  considered  to  underlie  the  first  named.  In  the  shaft  portions  were  passed 
through  which  resemble  the  Tonopah  rhyolite-dacite;  these  ma}'  represent  dikes, 
especiall}'  in  the  lower  portion.  As  a  whole,  however,  the  shaft  may  be  considered 
to  lie  within  the  Fraction  dacite  breccia. 

The  first  150  feet  is  rather  fine  volcanic  breccia,  followed  by  275  feet  of  frag- 
mental  tuff,  light-colored  and  generally  moderately  coarse.  This  is  horizontally 
coarsely  stratified  and  contains  one  bed,  1£  feet  thick,  of  finely  stratified  fine-grained 
material.  This  passes  gradually  into  a  fine  breccia  and  this  into  a  vecy  coarse 
breccia  containing  included  fragments  up  to  2  or  3  feet  iu  diameter.  Most  of 
these  inclusions  are  various  phases  of  the  later  andesite,  but  some  are  probably 
earlier  andesite.  Others  are  of  dacite  and  tuff,  much  like  the  matrix.  At  the 
time  of  the  writer's  examination  the  shaft  was  475  feet  deep,  and  had  passed 
through  50  feet  of  this  coarse  breccia.  Specimens  obtained  from  the  shaft  during 
its  progress  farther  downward  showed  that  it  remained  in  practically  the  same 
material,  some  of  the  included  bowlders  of  earlier  and  of  later  andesite  being 
several  feet  thick.  The  bottom  of  the  shaft  is  in  soft  dacite  that  contains  later 
andesite  inclusions.  This  dacite  is  much  like  that  which  caps  the  Fraction  shafts. 

The  stratified  tuffs  referred  to  do  not  belong  to  the  Siebert  tuffs  of  the  lake 
beds,  but  are  included  in  the  Fraction  dacite  breccia.  They  are  described  else- 
where as  the  interbreccia  tuffs,  and  are  found  in  the  upper  part  of  the  Fraction 
dacite  breccia  at  various  places  in  the  district.  The  great  thickness  of  the  Fraction 
dacite  breccia,  here  shown,  indicates  that  the  block  in  which  the  New  York  Tonopah 
lies  has  sunk  down  very  considerably  in  respect  to  the  blocks  farther  northeast — 
to  those,  for  example,  in  which  the  Fraction  shafts  are  situated.  The  breccias 
and  tuffs  of  the  New  York  Tonopah  are  considered  to  be  surface  formations,  formed 
chiefly  by  explosive  outbursts;  and  the  included  blocks  of  earlier  rocks  are  con- 
sidered to  be  fragments  hurled  out  of  the  volcanoes  at  the  time  of  the  explosions. 

FRACTION   EXTENSION  SHAFT. 
GEOLOGICAL   SECTION'. 

This  shaft  is  situated  at  the  south  base  of  Brougher  Mountain,  somewhat  over 
a  thousand  feet  northwest  of  the  New  York  Tonopah  shaft.  When  visited  by  the 
writer  it  was  approximately  300  feet  deep.  On  account  of  the  tight  lagging  the 
section  of  the  shaft  could  not  be  observed,  but  a  roughly  estimated  thickness  of  75 
or  80  feet  of  the  white  finely  stratified  tuffs  of  the  lake  beds  was  first  passed 
through.  Below  the  tuffs  the  whole  shaft  is  in  hard  gray  or  red  brecciated  lava, 
belonging  to  the  glass}7  Tonopah  rhyolite-dacite. 


202  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

FAULT. 

This  same  rhyolite-dacite  outcrops  about  35  feet  east  of  the  shaft,  on  the 
farther  slope  of  a  little  gull}-.  A  northeast  fault  running  along  this  gully  is  thus 
evidenced,  and  indeed  is  shown  farther  northeast  up  the  hill  slope.  By  this  fault 
the  block  in  which  the  Fraction  Extension  is  located  is  downthrown  in  respect  to 
that  on  the  east  side. 

TONOPAH  CITY  SHAFT. 
GEOLOGICAL   SECTION. 

The  Tonopah  City  shaft  lies  on  the  outskirts  of  the  town,  about  1,100  feet 
south  of  the  Fraction  No.  2.  It  was  driven  to  a  distance  of  500  feet  before  work 
was  stopped.  On  the  surface  at  this  point  is  a  very  thin  covering  of  black  glassy 
rhyolite  or  dacite  (latest  rhyolite-dacite  flow),  generally  only  a  few  feet  thick,  and 
often  broken  up  into  bowlders  rather  than  in  place. 

Practically  none  of  this  black  lava  is  exposed  in  the  shaft  itself,  the  first  solid 
formation  cut  being  the  Fraction  dacite  breccia.  The  upper  100  feet  of  this  was  a 
coarse  breccia,  evidently  detrital,  which  contained  large  and  small  inclusions,  mostly 
of  later  andesite.  From  100  to  300  feet  the  breccia  was  finer  grained  and  denser, 
and  apparently  had  an  explosive  origin,  being  full  of  small,  angular,  white  pulveru- 
lent fragments,  which  are  probably  decomposed  pumice.  At  a  depth  of  300  feet 
solid  Heller  dacite  (see  p.  37)  came  in  and  continued  for  200  feet  to  the  bottom  of 
the  shaft. 

At  a  depth  of  400  feet  in  the  shaft  this  dacite  was  observed  to  be  cut  by 
a  dike  of  exactly  similar  material,  the  only  difference  being  the  presence  in  the 
dike  of  a  greater  abundance  of  light-colored  intrusions.  This  dike  is  10  inches 
thick  and  has  a  N.  70°  W.  strike  and  a  dip  of  75°  to  the  northeast. 

INDICATED    DISPLACEMENT   OF   FAULT   BLOCKS. 

Since  neither  the  earlier  nor  the  later  andesite  was  encountered  in  this  shaft, 
and  the  dacite  breccia  is  so  much  thicker  than  in  the  Fraction  shaft  to  the 
north,  it  is  plain  that  the  fault  block  in  which  the  Tonopah  City  is  situated  is 
depressed  relatively  to  that  in  which  the  Fraction  shafts  lie. 

• 

OHIO  TONOPAH   SHAFT. 
DACITE   TUFFS   IN    SHAFT. 

The  Ohio  Tonopah  is  situated  about  l,f>00  feet  west  of  the  MacNamara  shaft. 
At  this  point  the  surface  formation  is  a  volcanic  tuff  due  to  dacitic  outbursts. 
Home  of  the  harder  portions  are  more  clearly  referable  to  the  glassy  Tonopah 
rhyolite-dacite,  while  other  portions,  especially  where  the  rock  is  softer,  approach 


OHIO    TONOPAH    SHAFT.  203 

more  closely  the  character  of  the  Fraction  dacite  breccia.  However,  as  has  been 
said  in  discussing  these  formations  in  general,  there  is  much  admixture,  and  on 
account  of  the  intimate  relation  of  the  two  lavas  the  tuffs  often  can  not  be, 
properly  distinguished  and  separated. 

The  shaft  is  at  present  about  770  feet  deep,  and  has  a  working  level  at 
756  feet.  Passing  downward  from  the  surface,  the  shaft  passed  through  a  con- 
siderable thickness  of  the  rhyolite-dacitic  tuffs  above  referred  to.  These  tuffs 
continue  down  to  about  485  feet.  They  are  usually  rather  soft;  under  the  micro- 
scope they  are  plainly  fragmental  and  are  little  assorted,  indicating  probably 
showers  of  detritus  from  volcanic  outbursts.  On  account  of  their  original  glassy 
character  and  their  subsequent  decomposition  (chiefly  kaolinization)  very  few 
definite  characteristics  can  be  distinguished.  A  specimen  of  one  of  the  harder 
portions,  at  396  feet,  however,  showed,  under  the  microscope,  a  glassy  ground- 
mass  with  phenocrysts  of  quartz,  striated  feldspar,  orthoclase,  and  altered  biotite. 
In  this  slide  the  chief  secondaiy  minerals  were  calcite  and  muscovite. 

LATER   ANUESITE   IN    SHAFT. 

From  about  485  feet  to  525  feet  there  is  andesite  having  the  appearance  of 
the  later  andesite.  Well-marked  slips  near  this  contact  indicate  that  it  is  very 
likely  a  fault  contact.  One  of  these  slips  had  a  north-south  strike,  and  a  westerly 
dip  of  50°. 

SOLID   TONOPAH    RHYOLITE-DACITE. 

Below  the  later  andesite,  from  525  feet  to  the  bottom  of  the  shaft,  comes  a 
dense,  siliceous  rock,  which  is  discussed  elsewhere  and  is  undoubtedly  referable 
to  the  Tonopah  rhyolite-dacite. 

At  the  contact  of  this  rock  with  the  overlying  andesite  movement  is  indicated 
by  the  presence  of  30  to  40  feet  of  ground-up  material,  which  contains  fragments 
of  hard  rock  and  occasionally  of  quartz.  The  dip  of  this  contact  is  northwest, 
at  an  angle  of  about  25C. 

At  the  756-foot  level  the  ground  has  been  extensively  explored  to  the  south, 
north,  and  east  by  drifting,  the  main  southeast  drift  running  about  700  feet  from 
the  shaft.  The  formation  is  almost  entirely  Tonopah  rhyolite-dacite,  character- 
istically showing  angular  white  fragments  in  a  dense  gray  groundmass.  The 
brecciation  indicated  by  these  fragments  took  place  before  the  cooling  of  the 
rock.  The  only  andesite  shown  on  the  level  is  a  small  patch  about  150  feet 
southeast  of  the  shaft.  This  is  a  biotite-andesite,  and  may  be  either  the  earlier 
or  the  later  andesite.  It  has  a  sharp  contact  with  the  rhyolite-dacite,  which  is 
probably  intrusive  into  it.  On  the  south  side  of  the  andesite  patch,  as  exposed 
in  the  drift,  the  contact  dips  north  at  an  angle  of  about  55°. 


204  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

CHARACTERISTICS   OF   THE    RHYOLITE-DACITE. 

The  specimens  of  the  rhj-olite-dacite  examined  microscopically  are  of  a  highly 
altered,  very  glass}'  lava.  The  groundmass  is  glassy,  often  kaolinized.  It  is  very 
abundant,  constituting  nearly  all  the  rock,  and  often  shows  marked  flow  structure. 
The  phenocrysts  are  rare  and  small,  and  consist  chiefly  of  short,  blunt  feldspars, 
biotite,  and  occasional  very  small  quartz  grains.  Some  of  the  feldspars  are 
striated.  The  feldspars  are  usually  almost  or  entirely  altered  to  kaolin,  sericite, 
and  secondary  adularia.  The  biotite  is  usually  altered,  sometimes  to  chlorite. 
Secondary  quartz  and  pyrite  are  usually  common  in  the  rock,  and  sometimes 
there  is  calcite. 

MINERALIZATION. 

As  a  rule  the  rock  is  very  much  silicilied.  Cracks  in  this  rock  are  filled 
with  coatings  of  calcite,  quartz,  and  pyrite,  and  excellent  free  crystals  of  barite. 
Some  streaks  are  considerably  silicitied,  and  contain  silver  and  gold,  as  is  shown 
by  assay.  Up  to  the  time  of  this  writing,  however,  no  veins  of  importance  have 
been  struck  in  this  formation. 

The  chief  veins  are  irregular,  barren,  and  nonpersistent.  They  have  a 
general  northeast  or  east  strike,  and  die  out  along  the  strike  by  scattering  into 
the  silicitied  rock,  or  are  cut  off  by  faulting.  At  the  upper  contact  of  the 
rhyolite-dacite  with  the  patch  of  andesite  above  mentioned,  there  are  2  feet  of 
jaspery  barren  quartz,  illustrating  again  the  tendency  of  the  rhyolitic  quartz  to 
form  at  the  upper  contact  of  the  rhyolite-dacite  body,  under  the  impervious 
decomposed  andesite,  as  elsewhere  described  in  the  discussion  of  the  MacNamara, 
the  Tonopah  Extension,  the  Mizpah  Extension,  and  other  mines. 

Some  faulting  is  shown  in  this  level,  the  chief  being  in  a  north-northwest 
direction,  and  indicating  in  places  considerable  displacement. 

PITTSBURG    SHAFT. 

The  Pittsburg  shaft  lies  near  the  eastern  edge  of  the  area  mapped,  on  the 
south  side  of  the  main  road  which  runs  east  out  of  Tonopah.  It  is  not  shown 
on  the  topographic  map,  having  been  started  since  this  was  made.  At  the  time 
of  the  writer's  visit,  in  November,  1904,  it  was  570  feet  deep,  all  in  volcanic 
breccia,  probably  belonging  chiefly  to  the  Tonopah  rhyolite-dacite  period.  This 
formation  contains  some  harder  layers,  which  may  be  flows,  but  as  a  whole  is  to 
be  considered  a  surface  formation,  the  product  of  volcanic  explosions. 

RED    ROCK    SHAFT. 

This  shaft  lies  about  halfway  between  the  Pittsburg  and  the  Ohio  Tonopah. 
It  was  at  the  time  of  the  writer's  visit,  in  November,  1904,  230  feet  deep  in 
volcanic  breccia  like  the  Pittsburg  and  the  upper  part  of  the  Ohio  Tonopah. 


DESCRIPTIVE   GEOLOGY    OF   MINES    AND   PROSPECTS.  205 

SHAFTS  ENTIRELY   OR  CHIEFLY   IN  LATER  AXDESITE. 

HALIFAX    SHAFT. 

The  Halifax  shaft  was  sunk  in  the  depression  lying  just  north  of  Golden 
Mountain  in  the  later  andesite,  just  northeast  of  a  probable  fault  line  which 
separates  the  later  andesite  on  the  northeast  from  the  white  tuffs  on  the  southwest. 
The  shaft  was  800  feet  deep  at  the  time  of  the  writer's  last  notes  in  November, 
1904,  and  was  entirely  in  the  later  andesite.  The  andesite  is  very  fresh — fresher 
than  that  examined  in  any  other  part  of  the  district.  The  phases  exposed  in  the 
upper  part  of  the  shaft  are  very  glassy,  suggesting  that  they  are  near  the  upper 
part  of  a  flow,  while  those  in  the  bottom  are  also  relatively  finer  grained  than 
the  rock  exposed  for  most  of  the  distance  down  the  shaft.  Much  of  this  latter  is 
so  coarse,  with  so  great  a  development  of  phenocrysts  compared  with  the  quantity 
of  the  groundmass,  that  it  has  in  the  hand  specimen  almost  a  granular  texture. 
Nevertheless  the  different  phases  all  belong  to  a  single  mass. 

At  a  depth  of  200  feet  in  the  shaft  a  drift  was  run  a  little  east  of  south  for 
270  feet,  and  in  the  opposite  direction  for  100  feet  along  a  heavy  fault,  which  runs 
parallel  to  the  drift  and  dips  west  at  an  angle  of  45°  or  steeper.  Along  this  fault 
plane  there  is  a  thick  brecciated  or  ground-up  zone  8  or  10  feet  thick.  The 
strise  indicate  that  the  faulting  was  normal,  the  downthrow  being  on  the  west 
side.  The  same  conclusion  is  suggested  by  the  difference  in  the  texture  of  the 
andesite  on  the  sides  of  the  two  fault,  that  on  the  foot  wall  side  being  coarser 
and  almost  granular,  while  that  on  the  hanging  wall  is  finer  grained.  It  is  probable 
that  the  coarser  textured  andesite  cooled  at  a  somewhat  greater  depth  than  the 
finer  grained  and,  therefore,  that  this  side  has  been  relatively  upthrust. 

The  shaft  stays  in  this  same  granular  andesite.  for  50  feet  below  this  level,  when 
another  fault  zone  comes  in,  along  which  is  also  a  clay  seam.  This  dips  60°  to 
the  west.  Below  this,  hard  and  finer  grained  andesite  comes  in  again  and  continues 
downward. 

This  is  one  of  the  few  shafts  in  the  district  which  have  struck  a  large  flow 
of  water.  Chiefly  below  600  feet  in  the  shaft,  water  was  encountered,  which  rapidly 
increased  from  10,000  to  30,000  gallons  a  day,  and  owing  to  this  the  sinking  of 
the  shaft  was  for  a  long  time  suspended  (see  p.  105).  Some  drifting  is  being  done, 
from  the  bottom,  north  and  south,  in  the  later  andesite. 

GOLDEN  ANCHOR  SHAFT. 

The  Golden  Anchor  shaft  was  started  in  the  center  of  the  later  andesite  area 
west  of  the  Midway.  When  last  visited,  in  the  middle  of  November,  1904,  it 
was  640  feet  deep.  At  a  depth  of  400  feet  a  south  crosscut  runs  510  feet  from 
the  shaft,  and  at  a  depth  of  500  feet  a  north  crosscut  runs  463  feet.  The  upper 


206  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,   NEVADA. 

part  of  the  shaft  is  in  typical  later  andesite.  On  the  400-foot  level  the  andesite 
is  of  somewhat  different  character,  being  greenish  and  altered,  but  its  characters 
still  indicate  that  it  is  probably  the  later  andesite.  On  this  level  it  contains  some 
calcite  veinlets,  but  no  quartz.  On  the  500-foot  level  the  andesite  is  finer  grained 
than  on  the  400,  and  has  some  of  the  features  of  the  earlier  andesite,  but  there 
is  little  doubt  that  it  belongs  to  the  same  body  as  the  400-foot  level,  and  the 
balance  of  evidence  is  therefore  in  favor  of  considering  it  probably  later  andesite. 
On  this  level  there  are  some  calcite  stringers  and  some  narrow  quartz  veinlets, 
containing,  however,  practically  no  values.  At  a  depth  of  550  feet  in  the  shaft 
a  change  of  formation  was  reported,  and  the  material  seen  on  the  dump  taken 
from  beneath  this  point  is  largely  a  dense,  green,  siliceous  rock  containing  quartz 
stringers.  A  specimen  of  this  examined  microscopically  proved  to  be  of  the 
glassy  Tonopah  rhyolite-dacite.  Inspection  of  the  dump  indicates  that  this 
rhyolite-dacite  is  mixed  with  some  andesite,  which  may  be  either  the  earlier  or 
the  later  andesite,  so  far  as  microscopic  characteristics  go. 

The  above  data  indicate  that  at  this  point  the  later  andesite  is  considerably 
thicker  than  in  the  territory  farther  east.      Indeed,  its  lower  limit  is  uncertain. 


CHAPTEE  VI. 

HOCK  ALTERATION. 

ALTERATION  OF   THE    EARLIER  ANDESITE. 

The  alteration  of  the  earlier  andesite  by  thermal  waters  has  been  profound, 
indicating  that  these  solutions  were  present  in  large  quantity  and  were  very 
active. 

ALTERATION     OF    EARLIER    ANDESITE    CHIEFLY    TO     QUARTZ,  SERICITE,  AND 

ADULARIA. 

On  Mizpah  Hill  the  andesite  is  entirely  altered  and  has  a  siliceous,  light- 
colored  rhyolitic  appearance  nearly  everywhere,  except  in  depth,  where  the 
Mizpah  shaft  on  the  700-foot  level  shows  earlier  andesite  altered  largely  to 
chlorite,  separated  from  the  quartz-sericite  alteration  by  a  fault,  and,  so  far  as 
yet  explored,  marked  by  the  absence  of  veins. 

ALTERATION   OF  HORNBLENDE   AND   BIOTITE. 

Various  stages  in  the  alterations  are  observable.  The  ferromagnesian  minerals, 
hornblende  and  biotite,  have  usually  been  completely  destroyed.  Their  areas  are 
marked  by  liberally  sprinkled  pyrite  crystals,  by  siderite,  and  often  by  some 
sericite.  Frequently  the  grouping  of  the  iron  minerals,  which  follows  with  more 
or  less  clearness  the  well-known  outlines  of  an  original  hornblende  or  biotite, 
affords  the  only  evidences  of  the  former  existence  of  these  phenocrysts,  at  the 
same  time  plainly  showing  the  demarcation  of  the  pyrite  and  siderite  from  the 
original  ferromagnesian  minerals.  In  further  stages  of  alteration  the  pyrite  and 
siderite  have  escaped  from  the  confines  of  the  original  crystal  and  are  scattered 
through  the  rock;  in  this  case  they  are  usually  less  abundant,  showing  a  leaching 
of  iron  out  of  the  rock  as  the  silicification  increases.  It  has  been  determined  by 
assay  that  the  pyrite  in  these  rocks  does  not  contain  appreciable  amounts  of  gold 
and  silver,  even  close  to  the  veins. 

In  other  phases  the  ferromagnesian  minerals  have  been  entirely  altered  to 
fine  muscovite  (sericite)  and  quartz. 

The  alteration  of  biotite  has  been  sometimes  not  so  complete  as  just  sketched, 
the  mineral  having  been  bleached  and  the  separated  iron  represented  by  pyrite  and 

siderite. 

207 


208  GEOLOGY    OF   TONOPAH    MINING    DISTRICT.   NEVADA. 

RELATIONS   OF    PTRITE    AND   SIDERITE. 

The  relations  of  the  siderite  to  the  pyrite  in  these  rocks  have  been  carefully 
studied.  In  some  cases  the  siderite  has  been  observed  distinctly  pseudomorphous 
after  the  pyrite.  Often  the  two  exist  side  by  side  in  such  a  way  as  to  suggest 
contemporaneous  deposition,  pyrite  showing  usually,  and  siderite  frequently,  some 
characteristic  forms  (PI.  XXIII).  In  observing  the  alteration  of  these  minerals 
from  ferrotnagnesian  crystals  it  has  been  repeatedly  noticed  that  the  carbonate  had 
more  intimate  relations  with  the  original  crystal  than  did  the  pyrite,  the  carbonate 
occurring  all  through  the  decomposed  mineral,  while  the  pyrite  was  distinctly 
confined  to  the  outer  zones. 

ALTERATION    OF   SODA-LIME    FELDSPAR   TO   QUARTZ    AND   SERICITE. 

The  feldspar  phenocrysts,  which  are  sometimes  fresh  enough  to  be  determined, 
are  typically  andesine-oligoclase,  though  sometimes  they  become  more  calcic. 
Labradorite  occasionallj'  occurs.  The}'  are  usually  partly  or  completely  altered. 

The  alteration  to  adularia  is  one  of  the  most  commonly  observed  changes,  but 
hardly  so  common  as  that  to  quartz  and  fine  muscovite.  These  two  last-named 
minerals  frequently  form  a  pseudomorphous  aggregate  in  the  space  occupied  by  the 
original  feldspar.  With  increasing  alteration  the  outlines  of  these  pseudomorphs 
become  more  and  more  indistinct  and  finally  indistinguishable.  Even  within  the 
veins,  however,  careful  observation  may  often  succeed  in  distinguishing  the  traces 
of  these  original  crystals  in  the  highly  silicified  mass,  for  sometimes  they  are  marked 
by  quartz  that  is  relatively  coarser  grained  than  that  in  the  groundnuts*,  and 
consequently  they  appear  slightly  lighter  in  transmitted  light.  These  two  processes 
of  alteration  of  the  feldspar,  either  to  adularia  or  to  quartz  and  sericite,  although 
present  in  the  same  rocks,  are  not  very  commonly  associated  in  the  same  specimens 
and  appear  to  be  distinct.  Occasionally  the  feldspar  is  altered  to  kaolin,  as  described 
later. 

ALTERATION    OF   SODA-LIME    FELDSVAR   TO   ADULARIA. 

The  alteration  of  the  soda-lime  feldspar  to  adularia  can  be  observed  in  all 
its  stages  in  different  rock  specimens.  The  alteration  proceeds  along  the  edges 
and  the  cleavage  cracks  of  the  crystal,  so  that  the  brightly  polarizing  andesine, 
somewhat  turbid  from  decomposition,  becomes  reticulated  with  the  fresh  glassy 
adularia,  which  shows  markedly  lower  polarization  colors  (PI.  XXIII).  Character- 
istic complete  or  incomplete  crystals  of  adularia  with  rhombic  outline  frequently 
form  within  the  older  crystal.  In  some  cases  the  alteration  is  completely  carried 
out  and  the  feldspar  is  completely  pseudomorphosed  to  adularia,  whose  perfect 
crystal  outlines  give  the  idea  of  a  fresh  primary  crystal,  but  whose  optical 


U.    S.    GEOLOGICAL    SURVEY 


PROFESSIONAL    PAPER    NO.   42      PL.   XXII 


A,    II,    C. 

D,  E,  F. 


RELATIONS  OF   PYRITE  AND   SIDERITE   IN   TONOPAH   ANDESITE. 
ADULARIA   IN    EARLY   ANDESITE  AND   VEINS. 


ALTERATION    OF    THE    EARLIER    ANDESITE.  209 

characters  prove  the  truth  of  the  change  demonstrated  in  other  cases  by  observed 
transitions.  Sometimes  the  alterations  to  adularia  and  to  sericite  go  on  side  by 
side,  the  original  feldspar  altering  in  part  to  one  and  in  part  to  the  other  and 
the  two  minerals  sometimes  forming  an  interlocking  aggregate. 

ALTERATION    OF  THE   GROUNDMASS. 

The  microlitic,  nearly  glassy  groundmass  has  been  very  largely  decomposed 
to  or  replaced  by  fine  granular  quartz,  with  fine  muscovite  (sericite),  etc.  The 
quartz  in  the  more  highly  silicitied  specimens  shows  grains  of  larger  growth  and 
is  often  segregated  in  bunches  or  veinlets.  Pyrite  and  siderite  are  very  commonly 
disseminated  throughout.  Original  zircon  is  frequently  present.  Sometimes 
adularia  can  be  made  out  as  a  portion  of  the  fine  secondary  aggregate.  Tiny 
veinlets  of  adularia  and  others  of  quartz  also  seam  the  rock. 

Apatite,  usually  brownish  or  yellowish  and  slightly  pleochroic,  is  relatively 
abundant,  and  not  being  easily  attacked  by  the  agents  which  have  brought  about 
the  alteration  of  the  rest  of  the  rock  is  very  characteristic  in  the  considerably 
silicified  phases. 

ADVANCED   STAGE    OF   ALTERATION. 

In  the  advanced  stages  of  alteration  nearly  all  the  iron  has  disappeared;  the 
similar  alteration  products  of  the  feldspars,  the  ferromagnesian  minerals,  and  the 
groundmass  merge  to  form  a  quartz-sericite  aggregate.  The  quartz  varies  in  grain 
from  microcrystalline  or  nearly  cryptocrystalline  to  moderately  coarse,  a  charac- 
teristic applying  also  to  the  quartz  of  the  mineral-bearing  veins,  which  are 
mostly  the  extreme  alteration  product  of  the  andesite,  as  is  shown  by  both  field 
and  microscopic  study.  In  these  extreme  phases  the  quantity  of  sericite  becomes 
less  and  that  of  the  quartz  more. 

OCCURRENCE    OF   KAOLIN. 

While  kaolin  is  not  an  ordinary  alteration  product  in  the  siliceous  alteration 
of  the  earlier  andesite,  it  is  frequently  present.  Specimens  in  which  it  has  been 
detected  have  usually  been  taken  from  near  a  fault  or  fracture,  or  other  water 
course  connecting  with  the  surface.  Therefore  the  hypothesis  has  been  formulated 
that  while  the  sericite  is  manifestly  the  work  of  the  vein-forming  solutions  the  kaolin 
is  the  work  of  descending  surface  waters,  and  is  probably  of  later  origin,  the 
kaolinization  attacking  the  unsericitized  residual  feldspar.  Kaolin  and  sericite 
are  frequently  found  together  in  varying  proportion. 
16843— No.  42—05 14 


210  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

ALTERATION    OF    EARLIER   ANDESITE,  CHIEFLY   TO    CALCITE    AND    CHLORITE. 

In  the  earlier  andesite  at  points  sufficiently  remote  from  the  important  veins, 
calcite  and  chlorite  appear  as  distinct  alteration  products,  which  do  not  occur  in 
the  rock  nearer  the  veins  and  which  take  the  place,  partly  or  wholly,  of  the  quartz 
and  sericite  of  the  phases  described  above.  This  phase  has  a  green  color,  growing 
in  depth  of  shade  as  the  proportion  of  chlorite  increases,  and  the  rock  has  no 
resemblance  to  the  light-colored  quartz-sericite  alteration  phases.  Iron  in  the  form 
of  pyrite  and  siderite  is  common  to  both  phases,  but  while  in  the  quartz-sericite 
alteration  it  is  characteristically  in  small  quantity  and  diminishes  with  increasing 
alteration,  in  the  chlorite-calcite  alteration  it  is  abundant  and  remains  so  when 
the  rock  is  completely  altered. 

In  this  process  of  alteration  the  feldspar  is  usualry  largely  altered,  chiefly  to 
calcite  with  a  little  quartz.  Rarely  the  alteration  is  to  quartz  and  epidote. 
Original  hornblende  and  pyroxene  are  always  completely  altered,  usually  to  chlorite 
(ripidolite)  pseudomorphs.  Biotite  has  been  observed  altered  to  sericite,  with  a 
little  calcite  and  hematite. 

The  groundmass  is  similarly  altered  to  chloritic  material,  intermixed  with 
secondary  quartz,  etc. 

TRANSITIONS  BETWEEN  ALTERATION   PHASES  OF  EARLIER  ANDESITE. 

There  are  all  transitions  between  the  typical  quartz-sericite  alteration  phase, 
in  which  calcite  and  chlorite  are  always  absent,  and  the  typical  calcite-chlorite 
phase,  in  which  quartz,  and  especially  sericite,  are  decidedly  subordinate.  Thus 
in  a  specimen  from  the  700-foot  level  of  the  Siebert  shaft  (from  the  same  rock 
mass  as  some  of  the  typical  calcite-chlorite  phases)  the  feldspars  are  chiefly  altered 
to  sericite,  with  a  little  chlorite;  the  hornblende  and  biotite  crystals  are  altered 
chiefly  to  chlorite;  and  while  calcite  is  present,  it  is  not  prominent. 

DIFFERENT  ALTERATIONS  THE  EFFECT  OF  THE  SAME  WATERS. 

The  conclusion  is  thus  reached  that  the  chemical  effects  of  the  same  mineralizing 
waters  became  continually  different  as  they  penetrated  to  a  greater  and  greater 
distance  from  the  circulation  channels.  Along  these  channels,  which  became  veins, 
the  transformation  or  replacement  of  the  rock  by  the  addition  of  silica  and  the 
sulphides  of  silver,  antimony,  etc.,  with  gold  and  selenides,  and  by  the  complete 
leaching  out  of  soda  and  magnesia  and  the  partial  leaching  out  of  lime  and  iron, 
was  profound.  In  the  siliceous  phase  of  the  altered  andesite  near  the  veins  a 
similar  alteration,  though  weaker,  is  recorded.  The  metals  did  not  penetrate  here, 
but  the  partial  replacement  of  lime,  iron,  magnesium,  and  soda  by  silica  and  potash 
is  present  in  all  its  stages.  In  the  rock  more  remote  from  the  vein  channels  the 


ALTERATION    OF    THE    EARLIER    ANDESITE.  211 

alteration  has  been  often  complete,  yet  there  has  been  no  very  great  increase 
or  decrease  in  the  original  elements.  The  original  combinations  of  these  elements 
have  been  broken  up,  and  hydrated  silicates,  with  abundant  carbonates  and  sulphides, 
have  formed,  indicating  only  the  presence  of  carbonic  acid  and  hydrogen  sulphide 
in  the  altering  waters.  Since  the  quartz-sericite  alteration  of  the  earlier  andesite 
grades  into  the  chlorite-calcite  alteration  by  all  possible  stages,  it  is  probable 
that  both  were  produced  at  the  same  time  and  by  the  same  waters;  and  since 
the  transition  from  the  quartz-sericite  alteration  to  the  metalliferous  quartz 
veins  is  similarly  perfect,  the  waters  are  clearly  those  which  have  produced  the 
mineralization.  Within  the  main  circulation  channels,  therefore,  these  waters 
introduced  silica,  potash,  and  the  metallic  sulphides,  and  abstracted  other  materials. 
As  they  penetrated  the  rock  away  from  these  channels  they  ceased  to  deposit 
metals,  except  possibly  in  trifling  quantity,"  while  the  excess  of  silica  and  potash 
was  still  deposited,  failing  with  increasing  distance.  Finally,  the  changes  in  the 
calcite-cblorite  alteration  show  that  only  the  common  gases  above  mentioned,  so 
commonly  present  in  surface  hot  springs,  were  left  in  the  mineralizing  waters, 
which  therefore  had  little  to  precipitate  and  small  power  to  abstract. 

The  successive  precipitation  so  plainly  demonstrated  probably  took  place  by 
reactions  with  the  wall  rock,  which  therefore  acted  as  a  screen  for  the  traversing 
solutions. 

REFRACTORINESS  OF  POTASH  FELDSPARS. 

In  arguing  that  the  formation  of  potash  minerals  in  the  veins  and  in  the  wall 
rock  shows  a  relative  excess  of  potash  in  the  mineralizing  waters,  it  must  be  taken 
into  consideration  that  potash  feldspars  are  ordinarily  more  refractor}'  to  altering 
waters  than  the  soda-lime  varieties.  Comparison  of  analyses  of  fresh  rocks  and 
of  rocks  altered  by  surface  weathering  usually  show  that  the  loss  of  soda  is 
greater  than  that  of  potash.*  It  is  also  true,  as  pointed  out  by  Lindgren,""  that 
one  of  the  most  prominent  minerals  formed  by  metasomatic  processes  in  and 
near  veins  is  a  potassium  mica,  such  as  muscovite,  and  that  the  most  prominent 
process  brought  about  by  the  waters  is  the  progressive  increase  of  potash  and 
the  decrease  of  soda.  At  the  Boulder  Hot  Springs,  described  by  Weed,rf  sericite 
and  in  one  case  adularia  had  been  deposited  from  the  waters,  which  contain 
chiefly  sodium  sulphate,  carbonate  arid  chloride,  calcium  carbonate,  and  silica; 
no  potassium  is  recorded.  Near  the  Comstock  lode,  potash,  as  compared  with 
soda,  is  more  important  in  the  altered  than  in  the  fresh  rock/  showing  that 

"Sampling  of  the  Mizpah  mine,  under  the  direction  of  Mr.  John  Hays  Hammond,  showed  that  the  earlier  andesite 
forming  the  walls  of  the  vein  runs  in  values  from  $0.50  to  $2  a  ton,  as  compared  with  many  times  that  value  in  the  vein. 
6  Merrill,  G.  M.,  Rocks,  Rock-weatliering,  and  Soils,  p.  236. 
eLindgren,  W.,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  30,  p.  690. 
dWeed,  W.  H.,  Twenty-first  Ann.  Kept.  U.  S.  Geol.  Survey,  pt.  2,  p.  246. 
«Lindgren,  W.,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  30.  p.  647. 


212  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

the  fresh  rock  has  been  attacked  more  than  the  altered  rock.  Complete  analyses 
of  the  mine  waters  show  chiefly  carbonates  of  lime  and  magnesia  in  about  equal 
proportions,  next,  sulphate  of  magnesia  and  silica,  a  smaller  amount  of  carbonate 
of  soda,  about  one-tenth  as  much  carbonate  of  potash  as  soda,  small  amounts  of 
sodium  chloride,  and  very  small  proportions  of  alumina  and  ferric  oxide." 

MEANING  OF  ADULARIA  AND  ALBITE  AS  GANGUE    MINERALS. 

While  it  might  be  inferred  from  this  that  ordinary  waters,  .even  those 
containing  a  large  amount  of  soda  and  little  or  no  potash,  tend  to  produce  potash 
minerals  in  veins  and  owe  their  composition  to  the  leaching  out  of  the  soda  while 
the  potash  is  left  behind,  the  fact  remains  that  potash  feldspar  is  contained,  so  far 
as  known,  only  in  a  relatively  limited  number  of  veins. 

Soda  feldspar  or  albite,  a  mineral  as  easily  formed  in  the  wet  way  as  orthoclase, 
occurs  in  a  number  of  other  veins  and  in  rocks  as  the  result  of  the  alteration  of 
soda-lime  feldspars,  and,  what  is  more  interesting,  of  potash  feldspars.  Dr.  G.  L. 
Gentil*  has  shown  that  in  the  granites  of  the  Tofna  basin  in  Algeria  the  soda-lime 
feldspars  have  been  largely  transformed  into  albite,  and  the  same  phenomenon  has 
been  described  by  other  authors.  On  St.  Gothard  and  other  places  in  the  Alps 
albite  has  been  described  as  pseudomorphous  after  adularia,  and  as  occurring  in 
porous  aggregates  of  fine  c^-stals  in  the  form  of  the  original  potash-feldspar 
crystal.  Comparative  analyses  of  the  feldspar's  various  phases  of  alteration  show 
that  the  original  adularia  contains  very  little  soda  and  the  resultant  albite  no 
potash.  Bischof  <"  explains  this  process  of  pseudomorphism  as  a  decomposition  of 
the  original  adularia  by  waters  into  a  perfectly  soda-free  adularia  and  a  potash-free 
albite.  The  potash,  silica,  alumina,  and  lime  of  the  adularia  were  dissolved  and 
carried  away,  leaving  the  albite;  in  some  cases  the  albite  substance  seems  to  have 
been  concentrated.  Bischof  suggests d  that  in  some  cases  part  of  the  adularia 
has  been  transformed  into  albite  by  replacement.  Also  in  localities  in  the 
Riesengebirge  in  Austria  small  fresh  albite  crystals  were  observed  in  several 
cases  upon  altered  orthoclase,  which  was  in  part  altered  to  muscovite/  Bischof 
and  Rose  agree  that  the  explanation  of  this  is  that  the  soda-feldspar  has  been 
abstracted  while  the  potash  feldspar  remains.  Bischof  remarks,  "  Such  opposite 
effects-'' presuppose  beyond  question,  if  not  opposite,  certainly  different  causes,  i.  e., 
different  substances  in  solution  in  the  waters." 


a  BecKer.  G.  F.,  Mon.,  U.  8.  Geol.  Survey,  vol.  3,  p.  162. 
'- 1 i.-nlil.  O.  L.,  Review  In  Am.  Geol.,  Apr.,  1903,  p.  264. 
oBlnchof,  Gustav,  Chemische  Geologic,  vol.  2,  p.  409. 
dOp.  clt.,  p.  412. 
«  Op.  cit.,  pp.  406, 407, 412. 

/That  In,  In  one  case  the  adularia  molecule  was  dissolved  out,  the  albite  molecule  being  insoluble;  in  the  other  the 
albite  molecule  was  diwolved  out,  while  the  adularia  molecule  was  insoluble. 


ALTERATION    OF   THE    EABLIER    ANDESITE.  213 

Bischof"  notes  that  albite  occurs  in  quartz  veins  in  gneiss  in  Sweden,  and 
F.  A.  Genth  described  it  in  pyritiferous  gold  quartz  veins  in  California,6  and  it 
has  been  noted  as  a  common  occurrence  by  subsequent  observers/ 

It  seems  to  the  writer  to  be  unquestionable  that  waters  that  deposit  albite 
without  orthoclase  in  a  vein  are  different  from  those  which  deposit  orthoclase 
without  albite,  and  that  the  difference  must  consist  in  part  in  the  relatively 
greater  quantity  of  soda  in  the  waters  in  the  first  case  -and  of  potash  in  the 
second.  The  many  observed  instances  in  the  earlier  andesite  at  Tonopah  of 
complete  pseudomorphs  of  adularia,  quartz,  sericite,  etc.,  after  soda-lime  feldspars 
show  a  process  of  replacement  (not  leaching  and  concentration),  the  soda  and  lime 
being  removed  and  potash  and  silica  introduced.  The  waters  which  accomplished 
these  changes  thus  must  have  had  abundant  potash  as  well  as  silica  in  solution. 

STUDY   OF    TYPICAL   SPECIMENS. 
MICROSCOPIC   DESCRIPTIONS. 

For  the  purpose  of  estimating  more  accurately  the  changes  which  have  been 
described  as  observed  microscopically,  a  number  of  analyses  were  made  and  studied. 
The  specimens  selected,  arranged  in  their  natui'al  order,  were  as  follows: 

1.  Earlier  andesite  (4O8)  from  Imoer  part  of  Siebert  shaft. — Dense  dark -green 
rock,    Siebert  shaft,    Mizpah   mine,   670  feet    from  surface.     Contains  scattered 
phenocrysts  of   rather  small  size  in  a  fine  microlitic  groundmass,  showing  flow 
structure.     The  microlites  in  the  groundmass  are  chiefly  feldspar.     A  little  zircon 
and  apatite  are  present.     Quartz  grains  also  occur,  of  which  some  may  be  original. 

Among  the  phenocrysts  the  feldspars  are  prominent.  A  determination  in 
another  similar  specimen  near  the  same  locality  showed  the  species  to  be  andesine- 
oligoclase.  They  are  largely  altered  to  calcite  with  a  little  quartz.  Abundant 
pseudomorphs  after  hornblende,  in  which  no  trace  of  the  original  mineral  remains, 
consist  of  dark  blue-green  chlorite  (ripidolite)  with  some  specular  iron.  The 
hornblende  cleavage  is  still  visible  in  the  pseudomorphs.  Pseudomorphs  after 
biotite  consist  of  fine  muscovite,  with  a  little  calcite  and  hematite. 

2.  Earlier  andesite  (358)  from.Tonopah  and  California  shaft. — Green,  but  much 
lighter   than    No.  1.     Shows   relatively   sparse   and   small   phenocrysts    in  a   fine 
microlitic    groundmass,  with    much    felty  devitrified    glass.     Apatite  is  abundant. 
Secondary  chlorite  occurs  throughout  the  groundmass. 

The  feldspar  phenocrysts  have  the  optical  characters  of  andesine,  and  are 
only  slightly  attacked  by  decomposition.  The  ferromagnesian  minerals  are 

«  Bischof,  Gustav,  Chemische  Geologic,  vol.  2,  p.  412. 

i>  Genth,  F  A.,  Am.  Jour.  Sci.,  2d  series,  vol.  28,  p.  249. 

c  Ransome,  F.  L.,  Description  of  Mother  Lode  district:  Geologic  Atlas  U.  S.,  folio  63,  V.  S.  Geol.  Survey,  1900,  p.  8. 


214  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,   NEVADA. 

entirely  altered;  pseudomorphs  of  chlorite  after  hornblende  can  be  distinguished. 
Numerous  amygdule-like  portions  are  lined  with  chlorite  and  filled  with  granular 
quartz. 

3.  Earlier  andesite  (293)  from  Fraction  No.  2  shaft,  at  depth  of  218  feet.— 
Purple  rock  with  vThite  feldspar  phenocrysts.  Phenocrysts  rather  abundant,  but 
relatively  small,  the  feldspars  being  the  largest.  The  groundmass  is  glassy  and 
microlitic,  with  flow  structure.  There  is  abundant  magnetite,  frequent  apatite, 
and  occasional  zircon. 

The  feldspars  were  determined  as  andesine;  they  are  only  partly  altered 
to  fine  muscovite.  Pseudomorphs  after  original  ferromagnesian  minerals  are 
abundant,  though  small;  biotite,  pyroxene,  and  hornblende  can  be  distinguished, 
though  no  traces  of  the  fresh  minerals  are  left.  The  biotite  has  altered  to 
muscovite,  with  a  small  amount  of  siderite  scattered  through,  and  hematite 
forming  a  zone  around  the  edge.  Rutile  cr  sagenite  needles  are  included  in  the 
biotite.  Pseudomorphs  after  hornblende  are  of  sericite  or  talc,  with  inclusions  and 
heavy  rims  of  magnetite.  Pseudomorphs  after  pyroxene  or  biotite  are  of  quartz, 
with  a  little  calcite  and  hematite  around  the  borders.  Other  pseudomorphs,  which 
are  probabty  after  hornblende,  but  may  be  in  part  after  pyroxene,  consist  of  quartz 
and  sericite. 

4-  Earlier  andesite  (53)  from  near  Mizpah  Hill. — Pale  pinkish-purple  ground- 
mass,  with  white  phenocrysts.  This  shows  what  was  originally  a  microlitic  glass}' 
groundmass,  now  containing  abundant  secondary  quartz  and  sericite,  with  dissemi- 
nated fine  limonite,  hematite,  and  siderite.  Pseudomorphs  after  biotite  phenocrysts 
are  of  muscovite,  with  a  very  little  siderite.  Other  phenocrysts,  possibly  of 
hornblende,  are  represented  by  pseudomorphs  of  quartz,  sericite,  and  a  little 
siderite.  Abundant  pseudomorphs  after  feldspar  are  of  clear,  translucent  mate- 
rial, which  appears  isotropic,  but  which  high  magnification  often  resolves  into  a 
fine  aggregate,  the  grains  of  which  may  sometimes  be  made  out  as  spherulitic. 
This  substance  has  a  very  low  double  refraction  and  also  a  low  single  refraction, 
but  the  latter  is  apparently  higher  than  that  of  balsam." 

5.  Earlier  andexite  (194)  from  Mispah  mine,  lease  86,  180-foot  level,  near  Mizpah 
vein. — Rock  of  a  light  salmon-pink  color.  Shows  several  phenocrysts  of  feldspar, 
whiter  than  the  rest  of  the  rock.  No  original  mineral  is  left  anywhere.  The 
groundmass,  of  which  the  fine  microlitic  glass}7  composition  and  fluidal  structure 

o  Some  of  this  material  was  Isolated  and  analyzed  by  Mr.  George  Steiger,  showing  62.1  per  c  ent  SiO«,  19  per  cent  A1SO3. 
and  4.8  per  cent  K2O.  Sodium  was  absent.  These  figures  correspond  to  a  composition  of  about  28. 4  percent  adularia,  30  per 
cent  kaolin,  and  27.5  per  cent  silica.  Water,  probably  contained  in  the  kaolin  and  the  silica,  was  not  determined,  and 
was  disregarded  In  the  computations.  The  substance  is  therefore  probably  to  be  regarded  as  a  colloidal  mixture  of  these 
three  alteration  products  of  the  original  soda-lime  feldspar,  In  nearly  equal  parts.  As  bearing  upon  the  change  which 
this  feldspar  has  undergone,  the  proportions  of  the  different  constituents  in  rather  siliceous  nndesinc,  such  as  we  may 
believe,  from  examination  of  fresher  rock,  that  this  altered  feldspar  originally  wns,  are  given:  SlOj,  60.36;  A1ZO3,  25.45; 
CaO,  5.14;  NajO,  7.63;  KSO,  1.21.  The  change  evidently  has  consisted  mainly  in  a  removal  of  the  soda  and  lime,  and  a 
substitution  in  part  of  exogenetlc  potash. 


ALTERATION    OF    THE    EARLIER    ANDES1TE.  '        215 

may  be  distinguished,  is  altered  to  an  aggregate  of  quartz  and  sericite,  with  a 
little  iron  oxide.  The  pseudomorph.s  after  phenocrysts  are  frequent  and  well 
denned.  Numerous  ones  after  feldspar  form  an  aggregate  of  fine  felty  muscovite, 
with  a  little  quartz.  Those  after  biotite  consist  of  muscovite,  with  a  little  siderite. 
Pseudomorphs  after  hornblende  or  pyroxene,  or  both,  are  barely  distinguishable 
from  the  groundmass.  They  consist  of  fine  muscovite  (sericite)  and  quartz,  with 
some  siderite,  which  marks  the  outlines  of  the  original  phenocrysts.  In  this  rock 
the  secondary  quartz  varies  in  grain,  some  areas  becoming  more  coarsely  crystalline. 

6.  Typical  earlier  andesite  (398)  from  Mizpah  Hill. — Hard  white   rock   with 
small  glistening  feldspar  crystals.     This  rock  has  a  microlitic  groundmass,  show- 
ing flow  structure.     It  has  the  appearance   of  being  unusually  fresh,  and  fresh 
striated  feldspar  can  be  seen  in   it.     Nests  of  fine  granular  adularia  and  quartz 
(both   secondary)   occur   here   and   there.     There   is   a   little    finely    disseminated 
siderite  and  limonite. 

The  feldspars  are  mostly  altered  to  adularia.  The  original  mineral  has  the 
optical  characters  of  an  oligoclase,  near  andesine.  The  alteration  of  this  to  adu- 
laria can  be  seen  in  all  its  stages.  Polarized  light  brings  out  this  change  strongly, 
the  bright  white  of  the  soda-lime  feldspar  contrasting  with  the  dark  gray  of  the 
potash  feldspar.  The  latter  penetrates  the  former  irregularly  and  minutely,  yet 
with  a  fairly  high  power  the  characteristic  crystal  outlines  (usually  rhombic)  of 
the  adularia  can  be  distinguished.  The  process  can  be  observed  in  all  its  stages 
in  different  crystals,  up  to  the  complete  pseudomorph.  A  little  sericite  accom- 
panies the  alteration  in  some  cases.  Traces  of  original  ferromagnesian  pheno- 
crysts can  be  determined,  but  with  difficulty.  In  one  case  a  pseudomorph  after 
probable  hornblende  was  of  sericite,  with  apparently  a  little  adularia  and  traces 
of  siderite. 

7.  Earlier  andesite  (143)  from  hanging  wall  of  Mizpah  vein,  300-foot  level, 
Mizpah  mine. — Light  gray,  nearly    white   rock,  with    uneven   fracture   and   dull 
luster. 

This  rock  is  so  much  altered  as  to  be  hardly  recognizable.  It  consists  of  an 
aggregate  of  quartz  and  fine  muscovite,  with  small  scattered  pseudomorphs  of 
hematite  after  pyrite  (the  result  of  oxidation),  and  some  siderite  (?).  The  quartz 
is  irregular  and  is  segregated  throughout  into  areas  and  little  veinlets,  which  are 
of  coarser  grain  than  the  quartz  of  the  less  altered  rocks,  while  the  muscovite  is 
apparently  finer  than  usual.  Original  phenocrysts  of  feldspar  are  indicated  by 
pseudomorphou  >  areas  characterized  by  different  groupings  of  the  quartz  and 
muscovite  and  freedom  from  iron,  while  others  of  ferromagnesian  minerals  are 
marked  by  similar  differences  of  grouping  and  by  a  relatively  greater  abundance 
of  the  iron  minerals.  In  all  cases,  however,  the  decomposition  products  are 
similar.  In  many  areas  also  the  vestiges  of  the  phenocrysts  have  been  effaced. 


216 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 


8.  Ore  matei'ial  (152)  of  Mizpah  vein,  300-foot  level,  west  drift. — Shows  in  the 
hand  specimen  dense  quartz,  intermixed  irregularly  with  apparently  kaolinic 
material. 

The  microscope  shows  tine  to  moderately  coarse  granular  and  retiform  quartz, 
with  much  fine  muscovite.  The  quartz  contains  inclusions.  Intermixed  with  the 
quartz  in  the  finer-grained  areas  is  adularia  in  characteristic  rhombic-sectioned 
crystals. 

ANALYSES    OF    DESCRIBED    TYPES. 

Following  are  the  analyses  of  these  rocks  by  Mr.  George  Steiger: 
Analyses  of  different  phases  of  altered  earlier  andesite. 


i. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

SiO2              

55.60 

58.47 

60.45 

71.14 

72.98 

73.50 

76  25 

91  40 

AI.O,  . 

16.70 

16.85 

17.78 

15.24 

14.66 

14.13 

12.84 

4  31 

Fe,0, 

2.23 

2.04 

5.86 

1.77 

1.01 

1  51 

54 

77 

FeO                   .          

3.51 

3.12 

.25 

.26 

.16 

.26 

33 

11 

MgO 

2.60 

3.84 

1.55 

.16 

.33 

.21 

.56 

18 

CaO      

4.27 

1.35 

1.04 

.09 

.18 

.12 

.  16 

Na,O  

4.08 

4.30 

3.58 

.24 

None. 

.24 

.  12 

06 

K.O.. 

3.17 

3.14 

2.11 

6.31 

6.03 

5.11 

3.20 

1.68 

H2O—                            

.88 

1.10 

2.86 

.85 

.97 

1.07 

2  14 

46 

H,O+          

3.06 

3.59 

2.93 

2.87 

2.95 

2.81 

3.17 

98 

TiO,          

.72 

.77 

.81 

.48 

.44 

.47 

.37 

.07 

ZrOj        

Undet. 

.02 

CO,  . 

2.76 

.52 

None. 

None. 

None. 

None. 

None. 

None. 

P,O< 

.28 

.35 

.28 

.05 

.16 

.09 

12 

04 

SO3                   

None. 

None. 

None. 

.05 

.17 

None 

Cl          

None. 

F      

.12 

Trace. 

S  

None. 

.02 

.03 

None. 

FeS, 

.49 

.06 

NiO  

(a) 

MnO    

Undet. 

.26 

(«) 

(a) 

(a) 

(«) 

(a) 

.06 

BaO  

.12 

.11 

.07 

.17 

(a) 

.19 

.02 

SrO  

(a) 

(a) 

(«) 

(a) 

(a) 

(a) 

(°) 

99.98 

100.42 

99.63 

99.70 

99.87 

99.91 

99.80 

100.16 

1.  Lower  part  of  Siebert  shaft. 

2.  Tonopah  and  California  shaft. 

3.  Fraction  No.  2  shaft. 

4.  Near  Mizpah  Hill. 

5.  Near  Mizpah  vein. 

6.  Mizpah  Hill. 

7.  Wall  of  Mizpah  vein. 

8.  Mizpah  vein. 


a  Not  looked  for. 


ALTERATION    OF   THE    EARLIER   ANDESITE.  217 

DIFFERENCES  OF  PHASES  EXPRESSED  BY  DIAGRAMS. 

The  changes  in  the  proportions  of  the  various  elements  in  the  rocks  can  be 
illustrated  by  diagrams  in  such  a  way  as  to  be  clearer  than  discussion.  In  tig.  73  the 
proportions  are  represented  by  straight  lines.  As  is  usual  and  more  accurate,  the 
proportions  plotted  are  the  quotient  figures  obtained  by  dividing  the  weight  per- 
centages by  the  molecular  weights.  The  scale  is  0.01  in  the  quotient  figure  =  one- 
fortieth  inch  (fig.  73). 

The  diagrammatic  lines  representing  the  different  elements  may  be  grouped 
together  for  each  analysis,  and  be  arranged  as  radii  of  a  circle,  with  lines  connecting 
the  ends  of  the  radii  to  form  an  irregular,  polygon,  forming  a  diagram  slightly 
modified  from  that  used  by  Brogger"  (PI.  XXIV). 

STUDY  OF  ALTERATIONS  INDICATED  BY  ANALYSES. 
ALTERATION    OF   EARLIER   ANDESITE    FROM    LOWER   PART   OF   SIEBERT   SHAFT. 

The  proportions  of  the  different  constituents  represented  by  the  diagrams  of 
rock  No.  1  (PI.  XXIV)  are  practically  identical  with  those  in  a  fresh  andesite.  That 
this  is  so  is  shown  by  the  diagram  («),  prepared  in  a  similar  way  to  those  referred 
to  above,  by  Prof.  W.  H.  Hobbs,  to  illustrate  the  typical  composition  of  andesite.6 
The  analysis  upon  which  this  diagram  was  based  was  obtained  by  averaging  seven 
analyses  of  mica  and  hornblende  andesites  from  the  Eureka  district,  Nevada;  Ouster 
County,  Colo.;  Cartagena,  Spain;  the  Siebengebirge  on  the  Rhine;  Panama;  and 
Colombia.  The  scale  of  the  diagram  has  been  adjusted  by  the  writer  to  correspond 
with  the  scale  of  his  own.  From  this  general  correspondence  it  becomes  apparent 
that  the  profound  alteration  which  rock  No.  1  has  undergone  has  resulted  in 
decomposing  the  original  minerals  and  changing  the  constituent  elements  to  new 
minerals  more  stable  under  the  new  conditions — that  is,  in  the  presence  of  the 
permeating  waters. 

a  Hobbs,  W.  H.,  Jour.  Geol.,  vol.  8,  pp.  1-31.  6 Op.  cit.,  p.  23. 


218 


GEOLOGY    Oif   TONOPAH   MINING    DISTRICT,  NEVADA. 


Silica 
Si02 


Potash 
K,0 


2  _ 

4  I 

5  

6 

7  _ 


Alumina 
AI203 


Iron   oxides 
FeOand 


2 

3  — 

4  _ 

5  . 

6  . 

8  '. 


Magnesia 
MgO 


i 

a 

4  ~ 

3  . 

6  . 

7  . 

8  . 


2  _ 

3  _ 

4  . 

6  . 

8' 


Soda 
Na20 


Lime 
CaO 


1  — 

2  _ 

4  ~ 

6  . 


Scale-.. 01  (quotient figure) "^5  inch 


Fig.  73.— Uiagrum  to  show  changes  in  amounts  of  commoner  elements  during  stages  of  alteration  of  earlier  andeslte. 


U.   8.  GEOLOGICAL    SUftVEV 


PROFESSIONAL  PAPER  NO.  «2     PL.  XXIV 


DIAGRAMS  TO  SHOW   CHANGES   IN   COMPOSITION   BROUGHT  ABOUT   BY  ALTERATION   OF   THE   EARLIER  AN  DESITE 


ALTERATION    OF    THE    EARLIER    ANDESITE. 


219 


A  similar  conclusion  is  reached  by  comparing  the  analysis  of  the  Tonopah 
rock  with  analyses  of  Eureka  and  Washoe  andesites.  For  the  purpose  of  com- 
parison, the  following  table  is  presented: 

Comparison  of  Tonopah  unth  Waihoe  and  Eureka  rocks. 


1.  Tonopah. 

2.  Average 
type  mica- 
hornblende- 
andesite. 

Washoe  rocks. 

Eureka  rocks. 

3.   Horn- 
blende-mica 
andesite, 
Mount  Rose. 

4.    Horn- 
blende-mica 
andesite, 
Cross  Spur 
quarry. 

5.  Mica- 
andesite, 
east  of  Wal- 
ler Defeat 
shaft." 

6.  Andesite- 
pearlite, 
south  of 
Carbon 
Ridge. 

7.  Pyroxene- 
andesite, 
Richmond 
Mountain.* 

SiO2        

55.60 
16.70 
2.23 
3.51 
2.60 
4.27 
4.08 
3.17 
3.94 
2.76 

62.16 
16.45 
3.27 
2.71 
2.20 
4.13 
4.07 
3.45 
1.15 

63.30 
17.81 
3.42 
.83 
2.07 
5.12 
4.27 
2.26 

63.13 
16.00 
4.34 
1.52 
2.07 
4.45 
3.87 
2.65 

65.68 
15.87 
1.78 
1.25 
1.79 
3.50 
3.20 
3.37 

65.13 
15>73 
2.24 
1.86 
1.49 
3.62 
2.93 
3.96 

61.58 
16.34 

A  1,O, 

Fe.O, 

FeO                   

6.42 
2.85 
5.13 
2.69 
3.65 

MgO 

CaO               

Na,O 

K,O.. 

H2O 

CO 

98.86 

99.59 

oTheae  are  the  designations  (riven  by  Hague,  Mon.  U.  S.  Geol.  Survey,  vol.  20,  p.  282.    The  designations  previously 
given  by  Becker,  Mon.  U.  S.  Geol.  Survey,  vol.  3,  p.  152,  are  3  and  4,  later  hornblende-andesite;  5,  mica-diorite. 
6  Mon.  U.  S.  Geol.  Survey,  vol.  20,  p.  264. 

The  difference  between  the  sums  of  the  first  two  analyses  is  largely  accounted 
for  by  the  difference  in  titanium,  of  which  the  Tonopah  rock  contains  0.72  per 
cent  and  the  average  rock  0.23  per  cent.  When  these  are  added  the  sums  are 
99.58  and  99.82  respectively. 

It  will  be  seen  that  there  is  a  remarkable  similarity  in  the  amounts  of  the 
bases  present  in  the  first  two  analyses.  In  the  Tonopah  rock  more  of  the  iron 
is  in  the  ferric  condition,  but  the  amounts  of  iron  in  the  two  rocks  are  almost 
identical. 

In  the  altered  Tonopah  rock  the  percentage  of  silica  is  about  6£  less  than  in  the 
average  type,  while  that  of  water  is  2f  greater.  The  Tonopah  rock  contains  2f  per 
cent  of  carbonic  acid,  which  is  lacking  in  the  average  type.  Thus  the  increase  of 
6£  in  the  percentage  of  water  and  carbonic  acid  in  the  Tonopah  rock  offsets  the 
increase  of  64  in  the  percentage  of  silica  in  the  average  type.  Since  free  primary 
quartz  is  apparently  rare  in  all  these  rocks,"  the  silica  is  combined  with  the  bases  to 
form  the  silicates,  feldspar,  hornblende,  and  mica;  and  since  the  amounts  of  the 
bases  are  equal  in  both  analyses,  the  original  amount  of  silica  was  probably  nearly 

a  For  the  Eureka  type  see  Arnold  Hague,  Mon.  U.  S.  Geol.  Survey,  vol.  20,  p.  234. 


220 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 


the  same;  that  is,  the  transformation  of  pyroxene,  hornblende,  and  mica  in  the 
Tonopah  rock  largely  to  calcite,  chlorite,  muscovite,  and  hematite  was  effected 
without  appreciable  gain  or  loss  of  the  bases,  but  some  of  the  silica  was  abstracted, 
its  place  being  taken  by  water  and  carbonic  acid,  which  entered  into  the  decompo- 
sition products  mentioned."  Therefore,  since  these  waters  abstracted  instead  of 
precipitating  silica,  they  were  characterized  by  relative  poverty  in  silica.  They 
were  also  carbonated.  The  lack  of  sulphur  and  sulphides  in  rock  No.  1  also  shows 
the  absence  of  sulphur  combinations  in  the  altering  waters. 

ALTERATION    OF    EARLIER    ANDESITE    FROM   CALIFORNIA    AND   TONOPAH    SHAFT. 

Tonopah  rock  No.  2  and  the  average  type  may  also  be  compared  as  to  their 
chief  constituents: 

Comparison  of  Tonapah  rock  No.  2  with  average  type. 


Tonopah 
rock  No.  a. 

Average 
type. 

SiO2  

58.47 

62.16 

A120,  

16  85 

16  45 

Fe2Os  

2  04 

3  27 

FeO  

3.12 

2  71 

MgO  

3.84 

2.20 

CaO  

1  35 

4  13 

Na,O  .  . 

4.30 

4  07 

K2O  

3.14 

3  45 

H2O  

4.96 

1.15 

CO, 

52 

FeS2 

49 

Total 

99  08 

99  59 

The  difference  between  the  totals  of  these  two  analyses  is  again  accounted 
for  mostly  by  the  difference  in  titanium,  the  percentage  of  which  in  the  Tonopah 
rock  is  0.77  and  in  the  average  rock  0.23.  When  these  are  added  the  total  of  the 
Tonopah  analysis  is  99.85  and  that  of  the  average  analysis  99.82. 

The  bases  present  correspond  very  closely,  the  only  noticeable  difference  being 
in  the  proportions  of  lime  and  magnesia.  In  the  average  type  the  percentage  of 
lime  is  three  times  as  much,  or  2.78  more,  and  that  of  magnesia  is  somewhat 
more  than  half  as  much,  or  1.64  less.  If  the  lime  and  magnesia  in  each  rock 
are  added  together  the  percentage  of  these  constituents  is  only  1.12  greater  in 

"It  1»  only  the  water  given  off  above  100°  C.  (HzO-(-  in  the  analyses,  p.  27)  which  can  be  considered  chemically 
combined.  The  rest(HaO-)  is  probably  mainly  hygroscopic,  mechanically  contained.  In  the  average  analysis  com- 
pared, aa  well  as  the  Washoe  und  Eureka  analyses,  however,  this  distinction  is  not  made.  Therefore  all  the  water  in 
the  Tonopah  rock*  Is  considered  together  in  comparing  with  these  analyses,  and  the  hygroscopic  water  in  one  is  -up 
posed  to  be  offset  by  that  in  the  other.  Most  of  the  water  in  the  Tonopah  analyses,  it  will  be  seen,  is  chemically 
combined. 


ALTERATION    OF    THK    EARLIER    ANDESITE. 


221 


the  average  type.  This  change  is  probably  due  to  the  alteration  of  the  horn- 
blende to  chlorite,  the  lime  being  in  part  carried  out  of  the  specimen  instead  of 
being  entirely  precipitated  in  place  as  carbonate,  its  place  being  taken  by 
magnesia.  These  changes  are,  however,  mainly  compensating,  and  probably 
indicate  a  local  rather  than  a  widespread  interchange.  Apart  from  this  the 
correspondence  of  the  bases  is  close.  In  the  average  fresh  type,  however,  the 
percentage  of  silica  is  3.69  greater  than  in  the  Tonopah  rock,  and  that  of 
water  is  3.80  less,  while  the  Tonopah  rock  contains  0.52  per  cent  carbonic  acid. 
The  conclusion  is  the  same  as  in  comparing  the  first  Tonopah  rock,  that  in  this 
second  specimen  there  is  an  increase  of  over  4  in  the  percentage  of  water  and 
carbonic  acid,  which  has  entered  into  the  composition  of  chlorite  and  calcite, 
while  this  gain  has  been  compensated  by  a  decrease  of  3.69  in  the  percentage 
of  silica.  The  process  of  alteration,  while  not  quite  so  far  advanced,  is  similar 
to  that  in  rock  No.  1,  except  that  the  lime  has  been  abstracted  and  com- 
pensated for  by  an  increase  in  magnesia.  The  presence  of  sulphur  in  the 
waters  is  indicated  by  the  relatively  small  amount  of  iron  oxide  which  has  been 
changed  to  pyrite,  a  change  which  did  not  take  place  in  rock  No.  1.  The  carbonic 
acid  present  is  only  a  fifth  of  that  in  rock  No.  1,  showing  that  in  the  case  of  rock 
No.  2  the  conditions  were  favorable  to  the  acid  acting  as  a  solvent  and  trans- 
porting the  lime  from  the  rock,  rather  than  as  a  precipitant  and  entering  into 
the  rock's  composition.  A  poverty  in  lime  in  the  circulating  waters  is  the 
apparent  explanation. 

ALTERATION   OF    EARLIER   ANUKSITE    FROM   FRACTION   NO.  2    SHAFT. 

The  comparison   between   rock   No.  3  and    the  average   fresh  type  may   be 

made  as  follows: 

Comparison  of  Tonapah  rock  No.  3  with  average  type. 


Tonopah 
rock  No.  3. 

Average 
type. 

Si02     

60.45 

62.  16 

Al2Oj 

17  78 

16  45 

Fe2O3  

5.86 

3.27 

FeO 

25 

2  71 

MgO  .  . 

1.55 

2.20 

CaO   

1.04 

4.13 

Na.2O 

3  58 

4  07 

K.O.. 

2.11 

3.45 

H,O 

5  79 

1  15 

CO,  

None. 

FeS, 

.06 

Total  

98.47 

99.59 

222  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

As  in  the  two  former  comparisons,  the  difference  in  the  titanium  determined 
accounts  for  most  of  the  difference  between  the  totals  of  these  analyses. 

In  this  case  the  bases  have  been  more  plainly  affected  than  in  the  first  two. 
The  most  noticeable  change  is,  as  before  (in  rock  No.  2),  the  abstraction  of  lime, 
whicli  seems  to  have  been  carried  farther  than  in  rock  No.  2.  Yet  in  this  case 
the  loss  has  not  been  compensated  for  by  the  deposition  of  magnesia — which  has 
itself  been  abstracted,  though  not  in  so  great  degree — so  that  the  combined  amount 
of  lime  and  magnesia  in  rock  No.  3  is  less  than  half  what  it  is  in  the  average 
type.  Simila-ly  the  alkalies  have  been  extracted;  the  potash  more  than  the  soda. 
The  iron  has  become  more  oxidized,  but  its  bulk  remains  the  same.  The  proportion 
of  alumina  has  slightly  increased,  perhaps  owing  to  the  loss  of  weight  of  the  rock, 
caused  by  the  removal  of  more  material  than  was  brought  to  replace  it.  In  all, 
the  percentages  of  lime,  magnesia,  and  the  alkalies  are  5.57  less  in  this  rock  than 
in  the  average  fresh  type.  There  is  also  less  silica,  but  the  difference  in  percentage 
is  by  no  means  so  great  as  it  was  in  rocks  No.  1  and  No.  2,  being  only  1.71. 
In  the  Tonopah  rock  (No.  3)  the  percentage  of  water  is  4.64  greater  than  in  the 
average  type,  carbonic  acid  is  absent,  and  there  is  a  very  small  amount  of  iron 
sulphide.  In  this  case,  therefore,  the  gain  in  water,  carbonic  acid,  etc.,  is  by  no 
means  offset  by  the  loss  of  silica.  The  chief  loss  is  plainly  lime,  magnesia,  and 
the  alkalies,  more  particularly  lime  and  next  to  that  potash.  In  this  case  the 
waters  have  extracted  silica  to  a  very  slight  extent  only,  and  were  therefore 
solutions  whose  silica  capacity  was  more  nearly  satisfied  than  in  the  case  of  rocks 
1  and  2.  The  tendency  to  dissolve  and  carry  away  lime— displayed  in  No.  2 — was 
more  vigorous  in  this  rock,  and  the  same  action  was  displayed  in  regard  to  magnesia 
and  the  alkalies. 


ROCK    ALTERATION. 
ALTERATION    OF   EARLIER    ANDESITE    FROM   NEAR   MIZPAH    HILL. 

Rock  No.  4  may  be  compared  with  the  average  type  thus: 

Comparison  of  Tonopah  rock  No.  4  with  average  type. 


223 


Tonopah 
rock  No.  4. 

Average 
type. 

SiO2 

71.  14 

62.16 

A12O3  

15.24 

16.45 

Fe,O, 

1.77 

3.27 

FeO  

.26 

2.71 

MgO  

.16 

2.20 

CaO 

.09 

4.13 

Na.,0  

.24 

4.07 

K..O 

6.31 

3.45 

H2O.. 

3.72 

1.15 

CO,  . 

None. 

Total  

98.93 

99.59 

In  this  rock,  as  in  rock  No.  3,  the  removal  of  magnesia,  iron,  and  soda 
has  gone  on  till  only  trifling  quantities  remain.  In  this  rock  also,  the  iron,  which 
was  relatively  free  from  attack  in  the  first  three  specimens,  has  been  partly  dis- 
solved, so  that  over  half  has  been  removed.  Even  the  difficultly  soluble  alumina 
has  apparently  lost  a  little,  though  this  is  doubtful.  Of  the  metallic  bases,  iron, 
lime,  magnesia,  and  alumina,  about  40"  per  cent  has  been  removed,  and  of  the 
same,  excluding  alumina,  about  73  per  cent.  On  the  other  hand,  while  soda  has 
diminished,  the  amount  of  potash  has  increased,  the  increase  of  one  nearly  com- 
pensating for  the  loss  of  the  other.  The  silica  also  has  increased  largely. 

In  this  case,  then,  the  waters  which  altered  the  rock  were  charged  with  an 
excess  of  silica  and  potash,  which  they  deposited,  attacking  and  dissolving  all 
the  other  components  of  the  rock,  the  relative  order  of  attack,  dependent  on 
their  solubility  in  the  attacking  waters,  being  lime,  magnesia,  soda,  iron,  and 
alumina. 


a  In  this  case,  as  in  many  of  the  similar  cases  in  the  following  pages,  the  percentages  given  are  in  terms  of  each 
constituent.  The  reader  will  notice,  however,  that  the  percentages  are  elsewhere  given  in  terms  of  the  entire  rock, 
where  such  presentation  has  best  lent  itself  to  statement.  This  is  the  case  with  all  of  the  figures  on  the  preceding  pages 
and  some  in  those  which  follow.  The  writer  believes  there  will  be  no  confusion  brought  about  by  the  use  of  these  two 
methods  of  presentation;  if  any  such  should  arise,  a  glance  at  the  compared  analyses  will  suffice  for  an  explanation. 


224 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 
ALTERATION   OF   EARLIER    ANDESITE    FROM    NEAR    MIZPAH   VEIN. 

The  alteration  of  No.  5  may  be  compared  as  follows: 

Comparison  of  Tonopah  rock  No.  5  with  arerage  type. 


Tonopah 
rock  No.  5. 

Average 
type. 

SiO2  

72.98 

62  16 

AL,OS  

14  66 

16  45 

Fe2O3  

1   01 

3  27 

FeO  

.  16 

2  71 

MgO  

.33 

2  20 

CaO  

18 

4  13 

Na2O  

None 

4  07 

K2O  

6.03 

3  45 

H2O  

3.92 

1.15 

Total  

99  27 

99  59 

Here  the  same  processes  have  been  carried  on  as  in  rock  No.  4,  but  more 
thoroughly.  As  in  No.  4,  only  tiny  amounts  of  the  magnesia  and  the  lime  are 
left,  while  the  soda  has  entirely  disappeared.  The  removal  of  the  more  refractory 
constituents — alumina  and  iron — has  apparently  proceeded  farther  than  in  No.  4. 
Of  the  iron  80  per  cent  has  been  removed,  against  70  per  cent  in  No.  4;  of  the 
alumina  about  11  per  cent,  as  compared  with  about  7  per  cent  in  No.  4.  On  the 
other  hand,  the  silica  has  increased  17  per  cent,  as  against  14  per  cent  in  No.  4. 
But  the  potash,  while  still  showing  an  increase  of  75  per  cent  over  the  normal 
proportion  in  the  type  analysis,  is  somewhat  less  than  in  No.  4.  It  appears  from 
this  (in  conjunction  with  the  succeeding  analyses)  that  the  increased  activity  of 
the  altering  solutions,  as  indicated  in  the  above  figures,  has  begun  to  attack 
some  of  the  introduced  potash  and  to  replace  it  by  silica,  or,  perhaps,  rather 
that  the  balance  is  more  in  favor  of  strong  silicification  than  of  the  introduction 
of  potash. 


ROCK    ALTERATION. 


225 


ALTERATION    OF  TYPICAL   EARLIER   ANDESITE    FROM   MIZPAH    HILL. 

The  relations  of  No.  6  are  as  follows: 

Comparison  of  Tonopah  rock  No.  6  with  arerage  type. 


Tonopah 
rock  No.  6. 

Average 
type. 

SiO2                                        

73.50 

62.16 

A  1,O,.. 

14.13 

16.45 

Fe,O, 

1.51 

3.27 

FeO 

.26 

2.71 

MgO  

.21 

2.20 

CaO 

.12 

4.13 

Na2O 

.24 

4.07 

K2O 

5.11 

3.45 

H2O 

3.88 

1.15 

Total  

98.96 

99.59 

This  shows  the  characteristic  alteration  of  No.  5.  with  some  further  advances. 
As  in  Nos.  4  and  5,  the  magnesia,  lime,  and  soda  are  almost  entirely  eliminated. 
The  alumina  is  further  reduced  than  in  No.  5,  14  per  cent  of  it  having  apparently 
been  abstracted,  while  the  iron  is  slightly  stronger.  The  decrease  of  the 
excessive  potash  to  make  room  for  the  increasing  silica  noted  in  No.  5  is  here 
carried  further,  No.  6  containing  0.92  per  cent  less  potash  than  No.  5,  and  0.52  per 
cent  more  silica  (in  proportion  of  the  whole  rock  composition). 

ALTERATION   OF   EARLIER   ANDESITE    FROM   WALL   OF   MIZPAH   VEIN. 

No.  7  may  be  compared  as  follows: 

Comparison  of  Tonopah  rock  No.  7  with  average  type. 


Tonopah 
rock  No.  7. 

Average 
type. 

SiOj              

76.25 

62.16 

A12O3 

12.84 

16.45 

Fe,O, 

.54 

3.27 

FeO 

.33 

2.71 

MeO 

.56 

2  20 

CaO        .... 

.16 

4.13 

Na2O 

.12 

4.07 

K2O  

3.20 

3.45 

H2O 

5.31 

1.15 

Total  

99.31 

99.59 

16843— No.  42—05 15 


226 


GEOLOGY   OF   TONOPAH   MINING    DISTRICT,   NEVADA. 


This  is  an  intensification  of  the  alteration  shown  in  the  immediately  preceding 
analyses.  The  lime,  magnesia,  and  soda  are  reduced  to  trifling  quantities.  The 
refractory  alumina  and  iron  are  further  reduced  than  before,  22  per  cent  of  the 
alumina  and  85  per  cent  of  the  iron  having  been  removed.  The  substitution  of 
silica  for  potash  (as  well  as  the  other  elements)  has  made  marked  progress,  the 
percentage  of  potash  being  1.91  less  than  in  No.  6,  and  that  of  silica  being  2.75 
per  cent  more.  In  this  way  the  excessive  potash,  caused  in  some  of  the  preceding 
cases  by  introduction  of  this  element  by  the  circulating  waters,  is  here  again 
brought  down  to  the  original  quantity. 

ALTERATION   OF   EARLIER  ANDESITE   TO  VEIN   MATERIAL. 

Rock  No.  8  may  be  compared  as  follows: 

Comparison  of  Tonopah  rock  No.  8  with  average  type. 


Tonopah 
rock  No.  8. 

Average 
type. 

SiO2                            

91.40 

62.16 

A12O3          

4.31 

16.45 

Fe,O, 

.77 

3.27 

FeO                                   

.11 

2.71 

MgO                      

.18 

2.20 

CaO        

None. 

4.13 

Na2O   

.16 

4.07 

K2O 

1.68 

3.45 

H20                              

1.44 

1.15 

Total                   

99.95 

99.59 

This  shows  the  further  continuation  of  the  changes  indicated  in  the  preceding 
analyses,  the  alumina  and  potash  being  gradually  removed  to  make  way  for  the 
increasing  silica. 

MAXIMUM  ALTERATION  LOCATED  ALONG  THE  VEIN   ZONES. 

The  specimens  thus  examined,  selected  as  being  fairly  well  representative, 
show  an  increasing  intensity  of  alteration,  beginning  with  only  a  slight  modifi- 
cation of  the  constituents  of  the  decomposed  rock  and  terminating  with  the 
intense  silicification  which  reaches  its  maximum  in  the  quartz  mineral-bearing 
vein  of  the  district.  Considering  the  alteration  from  the  standpoint  of  the  altering 
waters  rather  than  the  altered  rocks,  the  order  in  this  transition  series  is  reversed, 
for  these  changes  have  been  brought  about  by  solutions  which  circulated  along 
the  fracture  zones  which  are  now  largely  transformed  into  veins,  and  penetrated 
the  adjoining  rock  so  thoroughly  that  no  even  moderately  fresh  representative 


ALTERATION    OF    THE    EARLIER    ANDESITE.  227 

of  this  earlier  andesite  has  as  yet  been  found  in  Tonopah.  The  last  stage  of 
alteration  in  the  rock  (in  the  vein  zones)  was  then  in  a  sense  the  first  work  of 
the  waters,  and  the  first  stage,  remote  from  the  main  circulation  zones,  the  last; 
and  although  the  transition  as  studied  is  gradual,  it  by  no  means  follows  that 
the  rock  near  the  veins  went  through  all  of  the  stages  represented,  but  may 
have  reached  its  present  condition  much  more  directly. 

COMPOSITION  OF  MINERALIZING  WATERS  IN  THE  VEIN  ZONES. 

In  the  unoxidized  quartz  veins  the  predominating  gangue  mineral  is  quartz, 
with  frequent  adularia,  subordinate  muscovite  (sericite),  and  comparatively  rare 
carbonates  of  lime,  magnesia,  manganese,  and  iron.  The  metallic  minerals  are 
most  prominently  silver  sulphide,  containing  sometimes  antimony,  arsenic,  etc.; 
silver  selenide,  gold  in  some  form,  copper-iron  sulphide  (chalcopyrite),  iron 
sulphide,  and  probably  silver  chloride.  The  mineralizing  waters  were  then  charged 
with  an  excess  of  silica,  and  also  probably,  as  the  comparative  analyses  indicate,  of 
potash,  together  with  silver,  gold,  antimony,  arsenic,  copper,  lead,  zinc,  selenium, 
etc.  They  were  noticeably  deficient  in  iron,  since  they  have  removed  this  metal 
from  the  vein  zones  and  the  adjacent  rock,  more  and  more  completely  in  propor- 
tion as  their  work  has  been  thorough,  and  the  iron  left  in  the  veins  is  clearly  a 
residuum.  That  they  contained  carbonic  acid  and  sulphur  is  shown  by  the  for- 
mation of  sulphides  and  carbonates,  not  only  in  the  veins  but  in  the  altered  rock. 
That  they  contained  some  chlorine  and  fluorine,  though  not  in  excessive  amounts, 
is  indicated  by  the  presence  of  a  little  original  silver  chloride  and  by  their  work 
in  forming  muscovite,  as  will  presently  be  explained. 

In  the  vein  zone  the  maximum  effect  of  these  waters  was  a  replacement  of 
nearly  everything  by  precipitated  silica.  By  a  similar  process  of  replacement 
the  sulphides,  of  which  silver  sulphide  was  the  most  prominent,  were  precipitated, 
and  the  residue  of  the  comparatively  refractory  iron  was  combined  with  free 
sulphur  to  form  pyrite.  The  residue  of  the  comparatively  refractor}-  alumina 
combined  with  the  excessive  silica  and  potash  of  the  waters  to  form  adularia 
and  muscovite  (sericite). 

RELATION  OF  ADULARIA  TO  SERICITE  AS  ALTERATION  PRODUCTS. 

It  is  necessary  at  the  present  point  of  the  inquiry  to  investigate  the  conditions 
of  formation  of  adularia  and  of  muscovite.  Both  are  silicates  of  aluminum  and 
potassium,  and  both  are  conspicuous  as  secondary  products  in  the  altered  andesite, 
especially  of  the  feldspar.  The  typical  andesine-oligoclase  alters  sometimes  to 
adularia,  sometimes  to  quartz  and  muscovite,  sometimes  to  both.  That  one  of  these 
products  is  not  the  alteration  product  of  the  other  is  shown  by  the  fact  that  they 


228  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,  NEVADA. 

are  often  intercrystallized,  each  mineral  being  perfectly  fresh.  That,  however, 
they  depend  upon  slightly  different  conditions  for  their  formation  is  indicated  by 
the  fact  that  some  profoundly  altered  specimens  show  the  feldspar  almost  entirely 
altered  to  adularia  without  muscovite,  while  others  show  complete  alteration  to 
quartz  and  sericite  without  adularia.  Adularia  requires  more  silica  than  musco- 
vite, but  its  formation  in  preference  to  the  latter  does  not  necessarily  depend  on 
this  fact,  for  when  muscovite  is  formed  in  these  rocks  an  amount  of  free  quartz 
is  separated  out  equivalent  to  the  quantity  which  would  have  gone  into  the  adu- 
laria, as  is  shown  by  the  analyses  of  rocks  5,  6,  and  7,  of  which  5  and  7  are  altered 
chiefly  to  quartz  and  sericite,  and  6  chiefly  to  adularia.  This  difference  is  not 
shown  in  any  way  by  the  bulk  analysis  of  the  rocks,  the  relation  of  the  elements 
harmonizing  in  the  two  cases. 

FORMATION    AND    OCCURRENCE    OF    ADULARIA. 

CONDITIONS   REQUIRED   FOB  THE   FORMATION   OK  ADl'LARIA. 

Adularia  is  a  variety  of  orthoclase,  which  is  a  silicate  of  alum'num  and  potas- 
sium. It  is  distinguished  from  ordinary  orthoclase  chemical^  by  being  nearly 
pure,"  while  ordinary  orthoclase  contains  a  variable  and  often  large  amount  of 
soda.  Crystallographically  adularia  has  usually  an  entirely  different  habit  from 
ordinary  orthoclase,  and  this  crystallographic  difference  is  apparently  controlled 
by  the  difference  in  chemical  composition.  While  ordinary  orthoclase  is  one  of 
the  commonest  primary  minerals  in  igneous  rocks,  especially  in  the  more  siliceous 
varieties,  the  writer  is  not  aware  of  adularia  occurring  in  this  way.  On  the  other 
hand  it  is  known  as  a  secondary  mineral  in  metamorphosed  rocks  and  in  veins. 
Still,  experimental  investigations  do  not  seem  to  show  any  essential  difference  in 
the  conditions  of  formation. 

Orthoclase,  muscovite,  and  quartz  are  all  minerals  which  have  not  yet  been 
artitically  reproduced  by  the  cooling  of  dry  melts,  in  spite  of  many  careful  attempts.* 
All  these  may,  however,  be  formed  in  the  presence  of  such  agents  as  water,  chlorides, 
fluorides,  boron  compounds,  tungstic  acid,  etc.,  without  which  they  apparently  can 
not  crystallize.  These  agents,  so  potent  in  the  formation  of  minerals,  but  entering 
into  their  composition  slightly  or  not  at  all,  are  called  "mineralizers"  (agents 
mineralisateurs). 

Friedel  and  Sarasin  heated  a  mixture  of  potassium  carbonate,  alumina,  silica, 
and  water  in  a  platinum-lined  iron  tube  to  about  500°  C.,  for  fourteen  to  thirty -eight 
hours,  and  obtained  tiny  quartz  crystals  and  rhomboidal  tablets  of  feldspar.  Similar 
more  abundant  feldspar  crystals  were  obtained  by  heating  aluminum  chloride, 

"  It  usually  contains,  however,  a  little  soda,  lime,  etc. 

*  Vogt,  J.  H.  L.,  Mincralbildung  in  Silikatschmelzlosungen,  p.  6. 


ALTERATION    OF    THE    EARLIER    ANDESITE.  229 

potassium  silicate,  a  little  potassium  carbonate  and  water.  Analysis  showed  that 
this  mineral  had  the  composition  of  adularia  mixed  with  a  little  quartz.  The 
feldspar  crystals  were  sometimes  of  the  ordinary  orthoclase  habit,  and  sometimes  of 
the  adularia  habit."  The  same  investigators  obtained  orthoclase  crystals  by  heating 
potash,  silica,  and  muscovite  in  water  in  the  same  apparatus  as  mentioned  above, 
and  at  the  same  temperature. 

Calcite,  in  rhombohedral  crystals,  it  may  be  remarked,*  was  also  obtained 
under  similar  conditions  (temperature  500°  C.)  by  heating  precipitated  calcium 
carbonate  and  calcium  chloride  with  water  for  ten  hours. 


ADULARIA    AS    A    META.MORPHIf    MINERAL. 


Apart  from  the  primary  orthoclase  in  igneous  rocks,  secondary  orthoclase, 
due  beyond  question  to  attenuated  watery  solutions,  distinct  in  every  way  from 
rock  magmas,  has  been  often  described  as  occurring  in,  nature.  Van  Hise*1  showed 
that  clastic  grains  of  orthoclase  in  sandstones  on  the  north  shore  of  Lake  Huron 
had  been  enlarged  by  a  secondary  similarly  oriented  growth.  In  St.  Gotthard, 
in  the  Alps,  little  druses  in  a  fine-granular  quartz-albite  rock  contain  clear  crystals 
of  adularia  intercrystallized  with  calcite,  both  of  which  are  younger  than  the 
constituents  of  the  rock.  In  some  cases  the  adularia  is  provedly  younger  than 
the  calcite,  and  in  one  case  it  incloses  older  calcite  and  chlorite — both  water- 
formed  minerals — showing  that  the  feldspar  originated  as  a  precipitate  from 
solution. a  In  Chester  County,  Pa.,  orthoclase  occurs  in  dolomite,  indicating 
that  no  intense  heat  was  present  at  its  formation. d  In  the  metamorphosed  zones 
near  the  contact  of  intrusive  igneous  rocks  it  is  frequent,  as  was  shown  by 
Allport,  and  later  by  Teall,"  to  be  the  case  in  altered  lower  Silurian  slates  in 
England,  and  by  Lessen0  in  the  Harz. 


ADULARIA    IS    VEINS. 


Adularia  as  a  gangue  mineral  in  veins  has  also  been  described  a  number  of 
times.  In  a  vein  in  the  Herzog  Ulrich  mine  at  Kongsberg,  in  Norway,  Hausmann* 
found  adularia  with  quartz,  pyrite,  and  dolomite.  In  veins  in  Schenmitz,  in 
Hungary,  Wiser*  found  crystalline  adularia  associated  with  quartz,  dolomite,  pyrite, 
chalcopyrite,  blende,  and  gold.  In  the  Lake  Superior  copper  mines  orthoclase 
occurs  in  veins,  associated  with  calcite  and  native  copper;  the  feldspar,  like  the 
other  minerals,  is  plainly  formed  in  the  wet  way,  and  was  deposited  later  than 
the  copper  and  the  calcite.  Adularia  occurs  also  in  several  places  of  special 

a  Bull.  Soc.  francaise  de  min.,  vol.  4.  1881,  pp  171-175.  Chemisches  Centralblatt,  1892,  vol.  1,  p.  865. 
bin  connection  with  the  occurrence  of  calcite  and  adularia  in  the  same  veins  at  Tonopah. 
eCited  by  Zirkel.  Lehrbuch  d.  Petrographie,  vol.  1,  p.  243. 
d  Bischof,  Gustav,  Chemische  Geologie,  vol.  2.  p.  401. 
'Cited  by  Bischof,  Chemische  Geologie,  vol.  2.  pp.  898-399. 


230  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,  NEVADA. 

interest  because  of  their  geographic  and  geologic  relations  to  the  Tonopah  district. 
It  is  found  sparingly  in  the  Apollo  vein,  Unga  Island,  Alaska  (adularia  or  ortho- 
clase).  It  has  been  described  from  the  Valenciana  silver  mine,  in  the  State  of 
Guanajuato,  Mexico,  where  it  was  called  valencianite.  Lindgren  has  described 
it  as  a  common  gangue  mineral  in  the  Silver  City,  Idaho,  veins  (see  p.  272).  These 
ores  are  probably  post-Miocene,  and  Mr.  Lindgren  gives  reasons  for  considering 
that  the  deepest  ore  bodies  were  formed  at  a  distance  of  700  to  2,000  feet  below 
the  original  surface.  He  therefore  considers  that  the  temperature  at  the  time  the 
vein  was  formed  can  hardly  have  exceeded  100°  C.a 

At  Boulder  Hot  Springs,  Montana,6  are  springs  of  a  temperature  varying 
from  120°  to  164°  F/  which  contain  a  slight  amount  of  sulphureted  hydrogen, 
sodium  chloride,  soda  sulphate,  and  carbonates  of  soda,  lime,  and  magnesia. 
The  granite  through  which  they  rise  is  altered  in  the  vicinity  of  the  springs, 
the  most  notable  products  being  sericite  and  kaolinite,  the  result  of  the  alteration 
of  feldspar  and  quartz.  Calcite  does  not  occur  in  the  altered  rock,  and  has 
apparently  been  carried  out  of  it  by  the  altering  waters  into  the  fissures,  where 
it  has  been  deposited.  Veins  which  have  formed  in  this  granite  contain  chiefly 
medium-grained  quartz,  calcite,  and  stilbite,  and  a  little  adularia.  These  veins 
contain  slight  but  perceptible  amounts  of  gold  and  silver. 

At  Cripple  Creek,  Lindgren  found  that  adularia  has  been  formed  in  the 
granite  near  the  veins,  together  with  sericite  and  chlorite.  Within  cavities 
produced  by  the  removal  of  the  granite,  iron  pyrite,  fluorite,  and  tellurides  have 
been  deposited.* 

CHEMISTRY   OF  THE   ALTERATION   OF  SODA-LIME   FELDSPAR  TO   ADULARIA. 

The  chemistry  of  the  change  from  andesine-oligoclase  to  adularia  seems  to  be 
fully  explained  by  the  following  statements  of  Bischof,"  in  speaking  of  observed 
cases  where  adularia  was  altered  to  albite: 

"The  unequal  effect  of  water  upon  different  mineral  substances  is  mainly  based 
upon  the  fact  that  it  holds  materials  in  solution,  which  decompose  one  mineral  but 
not  another.  Sodium  chloride  decomposes  potassium  silicate,  and  potassium  chloride 
and  sodium  silicate  are  formed.  Thus  waters  which  hold  sodium  chloride  can 
decompose  potash  feldspar,  while  it  leaves  soda  feldspar  undecomposed. 

"In  this  way  it  is  possible  that  such  water  may  either  change  potash  feldspar  to 
soda  feldspar,  or  that  it  may  take  up  and  remove  the  alteration  products  of  the 
former.  We  can  realize  then  how  water  containing  sodium  chloride  (and  this  is 


a  Lindgren,  W.,  Twentieth  Ann.  Kept.  U.  8.  Geol.  Survey,  pt.  3,  pp.  165-167. 
6  Weed,  W.  H.,  Twenty-first  Ann.  Kept.  U.  8.  Geol.  Survey,  pt.  2,  pp.  236-248. 

e  By  personal  communication  with  Mr.  Weed  the  writer  learns  there  is  evidence  that  these  springs  reach  the  boiling 
temperature  not  many  feet  below  the  surface. 

<l  Lindgren,  W.,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  33,  p.  589 
'  Blue  hot,  Gustav,  Chemlsche  Geologic,  vol.  2,  p.  411. 


FORMATION   OF   MUSCOVITE.  231 

rarely  absent  in  waters)  breaks  up  the  potash  silicate  in  the  adularia  and  removes  the 
new-formed  soda  silicate  with  the  separated  alumina  silicate,  while  the  sodium 
silicate  contained  in  the  adularia,  with  the  combined  alumina  silicate,  remains  as 
albite. 

"  On  the  other  hand,  potassium  carbonate  decomposes  sodium  silicate.  It  is 
therefore  possible  that  water  containing  potassium  carbonate  may  either  transform 
soda  feldspar  into  potash  feldspar  or  that  the  alteration  products  of  the  former 
may  be  taken  up  and  removed.  Such  water  brings  about  the  opposite  of  that  in 
the  first  case.'"a 

This  explanation  corresponds  with  the  conclusion  as  to  the  excess  of  potash 
in  the  mineralizing  waters,  derived  from  a  comparison  of  the  rock  analyses. 

FORMATION   AND  OCCURRENCE   OF   MUSCOVITE. 

CONDITIONS   REQUIRED    FOR  THE   FORMATION   OF    MUSCOVITE. 

Muscovite,  as  previously  noted,  has  never  been  formed  artificially  by  cooling 
from  dry  fusion.  Concerning  its  formation,  as  well  as  that  of  other  micas,  Doelter 
observes:* 

"Mica  results  from  heating  aluminum  silicate  with  potassium  fluoride  or  mag- 
nesium fluoride;  the  fluorides  seem  to  assist  on  the  one  hand  because  the  fluoric 
vapors  which  form  bring  about  the  crystallization,  and  so  play  the  same  part  as  in 
the  transformation  of  amorphous  alumina  into  corundum;  on  the  other  hand  the 
influence  is  also  chemical,  since  small  quantities  of  fluorine  enter  into  the  composition 
of  the  mica." 

Brauns  remarks :c 

"Any  mica  can  be  easily  formed  if  one  melts  any  mineral  containing  its 
elements  with  any  fluoride  at  a  temperature  below  800°  C. ;  for  in  higher  temper- 
atures the  micas  are  not  stable." 

Of  the  micas,  biotite  or  magnesia  mica  is  found  in  many  volcanic  rocks,  such 
as  rhyolites,  dacites,  and  andesites,  while  muscovite  is  not;  neither  does  muscovite 
occur  in  the  deeper  seated  igneous  rocks  save  in  granites,  where  it  is  common, rf 
and  generally  occurs  together  with  quartz  and  potash  feldspar/  Evidently,  then, 
muscovite  demands  for  its  formation  special  conditions  not  present  in  lavas  or 
in  ordinary  rock  magmas  and  different  from  those  necessary  for  biotite. 

MUSCOVITE   AS  AN  ALTERATION   PRODUCT. 

Muscovite  is  common  as  a  secondary  mineral — the  alteration  product  of  many 
other  minerals,  such  as  feldspar,  nepheline,  leucite,  etc. — and  in  these  cases  is 
evidently  the  result  of  the  action  of  waters,  probably  heated.  It  is  very  abundant 

"The  italics  are  the  writer's  (J.  E.  S.). 

&  Doelter,  C.,  Allgemeine  Chemische  Mineralogie,  p.  161. 

o  Brauns,  R.,  Chemische  Mineralogie,  p.  247. 

dRosenbusch-Iddings,  Microscopical  Physiography  of  the  Rock-making  Minerals,  p.  2W. 

r  Brauns,  op.  cit.,  p.  301. 


232  GEOLOGY    OF  TONOPAH   MINING    DISTRICT,   NEVADA. 

in  the  metamorphic  rocks,  such  as  the  crystalline  schists.  It  forms  pseudomorphs 
after  orthoclase  in  tin  veins,"  where  it  is  associated  with  cassiterite,  tourmaline,  and 
quartz,  and  owes  its  origin  plainly  to  the  action  of  water  and  other  mineralizers, 
among  them  undoubtedly  fluorine;  near  the  veins  the  granite  is  entirely  altered 
to  a  mixture  of  quartz  and  muscovite  by  the  same  processes.  Weed  has  described 
it  as  produced  in  granite  by  the  action  of  hot-spring  waters  in  Montana.* 

DISTINCT  CONDITIONS  REQUIRED   FOR   MUSCOVITE   AND   FOR   BIOTITE. 

While  muscovite  is  the  alteration  product  of  so  many  minerals,  it  seems 
itself  not  at  all  subject  to  ordinary  alteration,  but  is  characteristically  fresh,  even 
in  highly  decomposed  rocks.''  Here  again  it  shows  its  distinction  from  biotite, 
which  in  rocks  traversed  by  waters  is  easily  altered  to  chlorite,  iron  oxides  and 
carbonates,  quartz,  epidote,  etc.,  showing  that  its  conditions  of  formation  were 
different.  In  many  granites,  indeed,  muscovite  and  biotite  have  been  found  side 
by  side,  and  in  these  rocks  the  conditions  for  the  formation  of  the  two  coincide, 
but  on  the  one  hand  stands  the  range  of  biotite  into  the  more  basic  igneous 
rocks  and  the  lavas  where  muscovite  does  not  occur,  and  on  the  other  is  the  range 
of  muscovite  among  the  minerals  formed  by  circulating  waters,  where  biotite 
does  not  ordinarily  occur.  Plainly,  then,  the  average  or  ordinary  conditions  under 
which  biotite  forms  are  more  heat  and  less  water  than  muscovite,  in  whose 
formation  the  evidence  of  comparatively  little  heat  and  abundant  water  is  often 
conclusive;  and  the  upper  extreme  of  the  muscovite  range  overlies  the  lower 
extreme  of  the  biotite  range  only  in  the  granites,  a  fact  which  affords  some  insight 
into  the  conditions  of  formation  of  this  rock. 

THE   8ERICITIC   VARIETY   OF   MUSCOVITE. 

The  fine-grained  muscovite  (which  is  often  the  secondary  product  of  other 
minerals  and  occurs  as  fine  fibrous  aggregates)  is  called  sericite.  Sericite,  however, 
does  not  differ  from  muscovite,  and  has  the  same  relation  to  the  coarser  variety 
(between  which  and  it  transitional  grades  of  coarseness  are  often  observable)  that 
the  tine  secondary  quartz  has  to  the  coarser  grains.  For  that  reason  the  author 
uses  the  words  muscovite  and  sericite  interchangeably  in  referring  to  the  tine- 
grained  variety. 

FLUORINE   NECESSARY  TO  THE   FORMATION   OF   MICA. 

Not  only  has  the  presence  of  fluorine  been  shown  to  be  necessaiy  for  the  artifi- 
cial reproduction  of  mica,  but  fluorine  enters  into  the  composition  of  the  mineral, 
Iwing  most  abundant  in  the  best  crystallized  varieties.**  The  sericitic  variety,  then, 

nRosenbiuch-Iddings,  Microscopical  Physiography  of  the  Rook-making  Minerals,  p.  286. 
''  Weed,  W.  H.,  Twenty-first  Ann.  Kept.  U.  S.  Oeol.  Survey,  pt.  2,  p.  247. 
<-Zlrkel,  Lehrbuch  d.  Petrographie,  vol.  1.  p.  340. 
rfBischof.  (justav,  Chemische  Geologic,  vol.  2,  p.  79. 


ALTERATION    OF    FELDSPAR   TO    SERICITE.  233 

m&y  be  assumed  to  have  crystallized  in  the  presence  of  a  less  potent  amount 
of  fluorine,  and  indeed  the  analyses  given  by  Dana"  do  not  show  any  fluorine,  while 
the  analyses  given  for  ordinary  rnuscovite  sometimes  do  and  sometimes  do  not  show 
it.  To  determine  its  presence  in  the  altered  Tonopah  andesite,  two  tests  for  it  were 
made,  in  No.  2  and  No.  8  (pp.  213,  216).  No.  2  showed  0.12  per  cent,  No.  8  a  trace. 
No  sericite  was  identified  in  No.  2,  while  No.  8  (the  vein)  contains  it.  The  tests 
therefore  are  not  convincing  as  to  the  fluorine  being  contained  in  the  mica,  but 
indicate  its  presence  in  the  waters  which  altered  the  rock.  No.  2,  it  may  be  noted, 
now  contains  between  three  and  four  times  as  much  water  as  No.  8.* 

CHEMISTRY   OF  THE   ALTERATION   OF  SODA-LIME   FELDSPAR  TO   SERICITE. 

The  alteration  of  soda-lime  feldspar  by  carbonated  waters,  according  to 
Rosenbusch,c  may  produce  calcite,  sericite,  and  quartz.  If  the  former  is  carried 
away  by  the  permeating  waters  only  quartz  and  sericite  results,  as  in  the  case  of 
orthoclase.rf  Where  orthoclase  is  similarly  altered,  some  potassium  carbonate  goes 
into  solution.  Similarly  Bischof  ^  suggests,  as  an  explanation  for  pseudomorphs 
consisting  largely  of  muscovite  (sericite)  after  feldspar,  such  as  he  describes, 
that  part  of  the  alkaline  silicates  of  the  feldspar  was  decomposed  by  carbonic 
acid,  their  silica  remaining  and  their  alkalies  being  removed  as  carbonates;  another 
part  of  the  silicate  was  removed  as  such;  and  the  rest  of  the  silicate  went  to 
form  the  mica.  In  this  way  a  mixture  of  mica  and  quartz  originated. 

The  analyses  of  sericite  pseudomorphs  after  feldspar,  given  by  Bischof  in 
connection  with  his  above-cited  explanation,  show  in  many  cases  the  presence  of 
fluorine;  whence  the  suggestion  arises  that  though  carbonic  acid  decomposes  the 
feldspar,  it  may  still  require  the  help  of  a  small  quantity  of  fluorine  for  the 
decomposition  products  to  crystallize  as  muscovite. 

CHANGES  IN  RARER    CONSTITUENTS    DURING    ALTERATION    OF    EARLIER 

ANDESITE. 

The  evidence  afforded  by  the  rarer  constituents  of  the  rock  is  less  trust- 
worthy, on  account  of  the  small  amounts  present.  The  percentages  of  titanium, 
barium,  and  phosphorus  in  the  different  rocks  are  represented  in  the  diagram 
forming  fig.  74,  the  scale  being  ten  times  that  employed  for  the  commoner  rock  con- 
stituents in  fig.  73  (p.  218).  It  is  here  seen  that  the  titanium  behaves  much  like 
the  alumina,  increasing  with  the  increasing  silica  in  the  first  three  specimens,  and 

"System  of  Mineralogy,  p.  618.. 

*  Fluorine  is  abundant  among  the  exhalations  of  cooling  igneous  rocks,  is  also  found  in  many  ordinary  waters,  in 
spring  waters,  and  even  in  sea  water.     (Bischof,  Gustav,  Chemische  Geologic,  vol.  2,  pp.  86-89.) 
c  Elemente  der  Gesteinslehre,  Stuttgart,  1898,  pp.  70-71. 
dThis  change  involves  the  substitution  of  potash  for  soda. 
t  Bischof,  Gustav,  Chemische  Geologic,  vol.  2,  p.  743. 


234  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,  NEVADA. 

slowly  decreasing  with  the  increasing  silica  in  the  last.  The  phosphorus,  though 
present  in  still  smaller  amounts,  behaves  in  much  the  same  way,  while  the  record 
of  the  barium  seems  irregular.  It  appears,  then,  that  even  the  resistant  rutile 
and  apatite  of  the  andesite  were  slowly  attacked  and  in  part  dissolved  by  the 
mineralizing  waters.  The  amount  of  combined  water  in  the  different  rocks  does 
not  vary  in  any  symmetrical  way,  and,  indeed,  remains  nearly  the  same  (about 
3  per  cent)  except  in  No.  8.  Carbonic  acid  was  noted  only  in  Nos.  1  and  2,  but 
microscopic  analysis  shows  that  siderite  is  usually  present,  often  in  very  small 
quantity,  in  most  of  the  other  rocks. 

RESUME  OF  EFFECTS  OF  MINERALIZING  WATERS. 

The  mineralizing  waters,    penetrating   vigorously    the   rock  on   each   side  of 
their  main  circulation  channels,  did  not  retain  their  metallic  contents,  which  wei'e 

all  deposited  in  favorable  places  in  the  main 
channels  or  in  special  lateral  channels  which 
became  lesser  veins.  However,  they  at- 
tacked the  rocks  vigorously  by  virtue  of 
the  carbonic  acid,  probably  also  sulphuric 
acid,  and  perhaps  to  a  less  extent  by  the 
acids  of  chlorine  and  fluorine.  The  ferro- 
magnesian  minerals  were  decomposed,  the 
lime  and  magnesia  were  taken  into  solution, 
and  the  iron  was  mainly  dissolved,  but  in 
part  was  altered  to  iron  sulphide  by  the  sul- 
phur in  the  waters.  The  feldspar  was  al- 
tered, probably  by  potassium  carbonate,  to 
adularia,  or  to  sericite  and  quartz,  the  lime 
and  soda  being  taken  into  solution.  Tocom- 
8 —  pensate  for  these  dissolved  materials,  silica 

Scale:.OI(qyotientfigure)-iinch  .       ,       „  ,  ,  .    ,  . 

F,<,74.-Diagram  showing  relative  proportions  of  the  WaS     deposited      from      the  highly    charged 

leas  common  elements  in  the  various  stages  of  altera-  waters.      So   great   W8S   the  llCCCSsitV  of  de- 
tion  ol  the  earlier  andesite. 

positing  the  silica  that  it  probably  takes  the 

place  of  part  of  the  alumina,  and  also  seems  to  have  replaced  even  part  of  the 
potash,  though  this  is  not  certain.  The  waters,  then,  after  passing  through  a  rock 
like  No.  8,  emerged  poorer  in  silica  and  richer  in  all  the  other  rock  constituents. 
On  passing  farther  and  traversing  a  rock  like  No.  7,  the  process  was  carried 
further,  though  the  excess  of  silica  was  not  so  great,  and  the  capacity  of  the 
solutions  for  the  different  rock  materials  became  somewhat  less.  Hence  the  least 
soluble,  such  as  the  alumina,  was  not  so  much  dissolved,  while  lime,  magnesia, 
and  soda  were  thoroughly  extracted.  On  passing  from  rocks  like  6,  5,  and  4  the 


CHANGES   IN   MINERALIZING    WATERS.  235 

process  is  continued  with  diminishing  strength.  Potash  here  is  thrown  down  by 
the  waters,  and  its  amount  is  greater  than  in  the  original  rock.  It  might  be 
argued  that  in  these  rocks  it  may  be  a  concentration,  and  that  its  percentage 
increase  is  only  apparent,  «and  is  due  to  its  remaining  constant  while  the  volume 
of  the  rock  increases;  but  the  decrease  in  the  similar  rocks  7  and  8  shows  that 
this  can  hardly  apply.  Again,  it  may  appear  that  the  increased  potash  in  the 
zone  represented  by  4,  5,  and  6  was  extracted  from  the  zone  represented  by 
7  and  8,  and  that  the  original  waters  did  not  necessarily  contain  much  potash; 
but  the  formation  in  the  main  vein  zones  of  often  large  proportions  of  potash 
minerals  bespeaks  an  original  large  amount  of  this  element,  as  noted  on  a 
preceding  page. 

CHANGES  IN  WATERS  AS  A  CONSEQUENCE  OF  ROCK  ALTERATION. 

The  waters  that  traversed  and  altered  this  broad  belt  of  rock,"  by  the  depo- 
sition of  silica  and  of  potash,  were  themselves  affected  by  the  interchange  and 
emerged  into  the  outer  zones  quite  transformed.  Rock  No.  3  indicates  that  they 
had  no  longer  any  excess  of  silica  and  that  their  solvent  power  was  much 
weaker.  Still  they  dissolved  part  of  the  lime  and  magnesia  in  the  rock,  as  well 
as  the  alkalies,  particularly  potash.  The  fact  that  they  dissolved  potash  shows 
that  they  no  longer  contained  an  excess  of  this  element.  Rock  No.  2,  still 
farther  removed  from  the  center  of  circulation,  shows  less  change  in  the  bases, 
the  alkalies  being  practically  unaltered.  The  lime  and  magnesia  have  been 
disturbed,  but  not  to  so  great  an  extent  as  in  No.  3.  As  in  No.  3.  much  of 
the  lime  has  been  extracted  (though  not  so  much  as  in  No.  3),  but  while  in  No. 
3  the  magnesia  also  has  been  extracted,  this  constituent  is  relatively  increased 
in  No.  4,  and  largely  compensates  for  the  loss  of  lime.  Here,  then,  the 
waters  replaced  some  lime  by  magnesia  and  abstracted  another  part.  The  analysis 
also  indicates  that  some  silica  was  abstracted.  By  this  time,  therefore,  the  waters 
had  so  effectually  precipitated  the  great  excess  of  silica  indicated  by  their  first 
effects  (as,  for  example,  in  No.  8)  that  they  were  now  able  to  take  up  fresh 
silica  from  the  rocks  which  they  traversed  instead  of  precipitating  it.  The 
presence  of  carbonic  acid  and  of  sulphur  is  indicated  by  the  pyrite  and  by  the 
analysis.  The  carbonic  acid,  though  undoubtedly  active  as  an  agent  in  the  altering 
processes,  was  in  the  more  highly  altered  types  so  hard  pressed  by  the  more 
urgent  silicification  that  it  was  free  to  form  very  little  carbonate;  but  on  the 

aOn  account  of  the  small  area  of  outcropping  earlier  andesite  at  Tonopah,  the  dimensions  of  these  zones,  such  as 
the  zone  ol  siliciflcation,  can  not  be  given  They  are  probably  variable.  The  earlier  andesite  outcrops  within  the 
limit  of  the  map  only  on  Mizpah  Hill  and  Gold  Hill,  covering  a  maximum  east-west  extent  of  over  2.000  feet.  Several 
veins  ouicrop  in  this  distance,  principally  on  Mizpah  Hill.  Nearly  all  of  this  andesite  is  siliciried  in  varying  degrees, 
the  less  altered  specimens  coming  principally  from  underground  workings  in  areas  where  the  andesite  does  not 
outcrop. 


236  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,   NEVADA. 

outer  edge  of  the  altered  zone,  as  in  No.  1,  the  case  was  different.  Here  calcite 
was  abundantly  formed  and,  with  abundant  chlorite,  makes  up  a  good  part  of 
the  rock  which  now  exhibits  the  typical  "propylitic"  alteration. 

PROI'YLITIC    ALTERATION    OF    EARLY   ANDESITE. 

Propylite  was  a  name  applied  in  1867  by  von  Richthofen  to  certain  early 
Tertiary  volcanic  rocks  of  Nevada  and  California,  especially  to  rocks  observed  near 
the  Comstock  lode  in  Nevada.  It  was  defined  as  being  always  porphyritic,  and 
very  similar  to  porphyritic  diorite,  with  oligoclase  feldspars  and  dark-green  fibrous 
hornblendes,  in  a  green  groundmass  which  owes  its  color  to  small  particles  of  fibrous 
hornblende;  as  being  very  rich  in  mineral  veins,  and  the  earliest  of  the  Tertiary 
volcanic  rocks.  These  definitions  were  accepted  and  new  areas  of  propylite  were 
discovered  by  many  prominent  geologists.  But  Dr.  G.  F.  Becker's  work,  published 
in  1882,  showed  that  the  "propylites"  near  the  Comstock  were  altered  rocks 
originally  identical  with  fresh  diorites,  andesites,  etc.,  from  the  same  region;  that 
the  characteristic  supposed  green  fibrous  hornblende  was  chlorite,  a  decomposition 
product;  and  that  this  rock  phase  owed  its  association  with  mineral  veins  to  the 
altering  mineral  waters  which  produced  the  veins  and  this  rock  at  the  same  time.0 
Other  investigators  have  come  to  the  same  opinion,  and  the  name  propylite,  as 
signifying  a  rock  type,  has  been  dropped.  It  has,  however,  been  sometimes  used  to 
signify  this  especial  form  of  alteration,  and  is  in  this  sense  characterized  by  Rosen- 
busch  as  follows:6 

''The  characteristic  feature  of  the  propylitic  facies  consists  in  the  loss  of  the 
glassy  habit  of  the  feldspars;  in  the  chloritic  alteration  of  the  hornblende,  biotite, 
and  pyroxene  (often  with  an  intermediate  stage  of  uralite),  with  simultaneous 
development  of  epidote;  further,  in  alteration  of  the  normal  groundmass  into 
holocrystalline  granular  aggregates  of  feldspar,  quartz,  chlorite,  epidote,  and  calcite, 
and  in  a  considerable  development  of  sulphides  (usually  pyrite)." 

Epidote  has  not  been  detected  in  the  earlier  andesite  at  Tonopah,  and  is  rare  in 
the  district  in  general;  otherwise  the  rocks  like  1  and  2  correspond  to  the  "propy- 
litic" phase.  At  the  Comstock  Becker c  found  epidote  uncommon  underground, 
while  abundant  at  the  surface. 

Mr.  Waldemar  Lindgren''  has  considered  gold  and  silver  veins  accompanied  by 
a  "propylitic"  alteration  of  the  wall  rock  as  a  group,  and  has  separated  them  from 
another  class  (the  sericitic  and  kaolimtic  gold  and  silver  veins)  whose  wall  rocks 
show  characteristic  alteration  to  sencite  and  kaolin.  In  a  subsequent  note  he 
remarks  that  "it  is  perhaps  not  advisable  *  *  to  retain  the  name  propylitic 
for  the  whole  group,  a.s  some  of  them  do  not  show  alteration  in  typical  form.'' 

a  Mon.  U.  S.  Geol.  Survey,  vol.  »,  p  88,  etc.  •'  Trans.  Am.  lust   Mln.  Eng.,  vol.  30.  pp.  645-664.  668-666. 

fcElemenlederGcstelnslehre,  Stuttgart.  1898,  p.  302.  'Ibid  ,  vol.  33.  p  798 

o  Mon.  V.  S.  Geol.  Survey,  vol.  4,  p  212. 


ALTERATION    OF    THE    EARLIER    ANDESITE.  287 

With  this  last  conclusion  the  writer  is  in  accord,  for  the  Tonopah  district  seems  to 
show  clearly  that  the  distinctions  between  the  two  classes  of  veins  are  artificial,  the 
predominating  alteration  of  the  wall  rock,  whether  to  sericite  and  quartz,  or  to 
chlorite,  calcite,  etc.,  depending  not  so  much  upon  the  original  character  of  the 
wall  rock  or  the  waters,  as  upon  the  abundance  and  intensity  of  the  latter,  and  on  the 
size  of  the  circulation  channels;  and  in  each  case  the  vein  materials  may  be  the  same. 
The  writer  has  already  pointed  out  the  close  analogy  of  the  Comstock  and  some 
other  districts  to  the  Tonopah  district;  in  some  of  the  districts  the  one  phase  of 
alteration  is  especially  represented,  in  others  the  opposite  extreme. 

FINAL,   COMPOSITION   OF   MINERALIZING    WATERS. 

The  waters  which  accomplished  the  "propylitic"  alteration  of  Nos.  1  and  2, 
therefore,  were  capable  by  virtue  of  their  carbonic  acid,  etc.,  of  decomposing  the 
original  minerals  and  forming  new  carbonated  and  hydrated  minerals  which  were 
more  stable  under  the  new  conditions.  They  were  not  able  to  remove  any  large 
quantities  of  the  bases,  with  the  exception  of  a  slight  amount  of  lime,  magnesia, 
and  silica,  and  of  the  alkalies.  The  character  of  such  waters  would  then  be  very 
different  from  what  it  was  when  they  were  fresh  from  their  channels  of  active 
circulation.  They  were  at  first,  if  the  reasoning  is  correct,  highly  charged  with 
silica  and  potash,  with  some  carbonic  acid  and  sulphur,  and  with  silver  and 
gold  and  relatively  small  quantities  of  other  metals.  They  would  finally,  as  a 
result  of  their  interchange  with  the  rocks  which  they  have  so  profoundly  altered, 
be  less  highly  charged  with  mineral  substances  and  would  contain  soda  largely  in 
excess  of  potash,  important  amounts  of  lime  and  magnesia,  some  iron,  a  little  silica, 
and  a  very  little  alumina;  and  at  the  best  only  traces  of  the  rarer  metals.  The  wall 
rock  in  fact  has,  by  its  reactions  with  the  mineralizing  solutions,  acted  as  a  screen, 
and  has  separated  successively  the  different  constituents  from  the  waters.  Similar 
phenomena  have  been  previously  observed,  and  a  chemico-physical  explanation  (the 
hypothesis  of  osmotic  action)  has  been  offered."  Dr.  G.  F.  Becker  remarks: 

"On  this  hypothesis  the  concentration  of  ores  in  deposits  would  be  largely  due 
to  the  fact  of  the  lack  of  action  between  their  solutions  and  the  wall  rocks;  and  the 
decomposition  of  the  country  rock,  so  often  observed  near  veins,  would  be  due  to 
the  absorption  of  solutions  of  gangue  minerals  by  the  walls.  In  short,  there  would 
be  a  species  of  concentration  by  dialysis."6 

The  writer's  explanation,  however,  as  indicated  above,  is  of  a  purely  chemical 
character.  He  assumes  that  the  ores  of  the  veins  did  not  penetrate  far  into 
the  wall  rocks  because  they  were  all  immediately  precipitated  in  the  main  cir- 

oBecker,  G.  P.,  Mineral  Resources  U.  S.  for  1892,  D.  S.  Geol.  Survey,  p.  166;  Eighteenth  Ann.  Kept.  U.  S.  Geol.  Survey, 
pt.  3,  p.  68.    Lindgren,  W.,  Trans.  Am.  Inst.  Mln.  Eng.,  vol.  30,  p.  691. 
i>  Mineral  Resources  U.  S.  for  1892,  U.  S.  Geol.  Survey,  p.  157. 


238  .  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

dilation  channels,  just  as  the  excessive  silica  did  not  penetrate  to  the  "pro- 
pylite"  belt  of  the  andesite  because  it  was  precipitated  before  it  arrived  there. 
The  evidence,  elsewhere  offered,  that  the  veins  themselves  have  formed  chiefly 
by  replacement  is  plainly  in  favor  of  the  writer's  explanation. 

If  such  changes  take  place  within  a  space  of  a  few  hundred  yards,  more  or 
less,  laterally  from  main  circulation  channels,  they  must  take  place  also  along 
those  channels  upward  (though  they  would  require  a  much  greater  distance),  for 
such  veins  as  those  at  Tonopah,  where  the  channels  were  for  the  most  part  not 
open  fissures,  but  only  zones  of  maximum  fracturing  in  the  rock,  and  the  vein 
formation  involved  intense  replacement  and  interchange.  When  the  waters  which 
accomplished  this  change  emerged  above  they  would  be  in  the  transformed  condi- 
tion described  for  the  lateral  moving  waters  emerging  from  the  propylitic  stage  of 
alteration — that  is,  they  would  resemble  the  waters  of  many  hot  springs,  or  the  hot 
mine  waters  of  the  Comstock  (see  p.  212).  It  is  not  necessarily  true  that  springs, 
even  hot  springs,  associated  with  mineral  deposits  have  a  composition  similar  to 
that  of  the  mineralizing  waters.  As  the  mineralized  area  is  eroded  the  critical 
area  for  mineralization  will  in  many  cases  probably  retreat  lower  down,  and  the 
same  interchange  between  water  and  rock  will  be  effected  at  a  lower  level.  When 
such  water  reaches  the  surface,  after  flowing  through  and  being  again  to  some 
degree  affected  by  the  ores  and  the  altered  rock  (which  were  stable  under  the 
conditions  of  original  deposition,  but  now  under  different  conditions  are  subject 
to  solution  and  redeposition),  it  will  still  contain  the  solutions  resulting  from  the 
mineralizing  reactions,  rather  than  those  which  accomplished  the  mineralization. 
This  may  perhaps  explain  in  part  why,  although  the  formation  of  veins  by  hot 
springs  has  in  man}'  cases  been  pretty  satisfactorily  demonstrated,  and  many  such 
springs  emerge  at  the  surface  at  the  boiling  point  or  over,  no  satisfactory  observa- 
tion has  as  yet  been  made  of  such  a  spring  depositing  near  its  exit  a  definite  and 

typical  vein. 

AT/TERATION  OF  THE  LATER  ANDESITE. 

The  later  andesite  is  not  altered  as  much  as  the  earlier  andesite;  it  outcrops 
over  a  much  greater  area,  and  is  often  found  nearly  fresh,  save  for  the  processes  of 
surface  weathering,  under  which  it  disintegrates  and  decomposes  easily.  At  many 
places,  both  at  the  surface  or  underground,  it  is  greatly  decomposed.  This  alteration 
is  extremely  irregular. 

STUDY  OF  TYPICAL  SPECIMENS. 

Four  analyses  have  been  made  to  show  the  composition  and  alteration  of  the 
later  andesite.  The  rocks  analyzed  are  described  as  follows: 

1.  Nearly  fresh  later  andesite  (225)  from  Mizpah  Extension  xhaft,  2^5  feet 
down. — Rock  nearly  black,  dense,  and  basaltic  looking.  A  very  dark  green  dense 


ALTERATION    OF    THE    LATER    ANDESITE.  239 

groundiuass  shows  fresh  crystals  of  feldspar  and  augite  largely  altered  to 
serpentine. 

Under  the  microscope  the  groundmass  is  seen  to  be  densely  packed  with 
microlites  of  feldspar  and  augite  partly  altered  in  the  same  characteristic  way  as 
the  phenocrysts,  which  are  to  be  next  described.  Magnetite  is  plentiful.  Siderite 
•  in  small  specks  is  scattered  throughout  in  characteristic  cloudy,  semitransparent 
white  aggregates.  Sometimes  this  mineral  forms  a  rim  around  the  magnetite, 
showing  derivation  from  it.  In  some  cases  there  may  be  discerned  characteristic 
rhombic  cleavage  and  even  rhombic  crystal  outlines. 

The  phenocrysts  vary  in  size  from  the  microlites  up  to  occasionally  moderately 
large  crystals.  They  are  of  feldspar  and  colorless  augite. 

The  feldspar  is  in  general  remarkably  fresh.  It  is  usually  striated,  and  is 
sometimes  in  complex  forms.  Two  optical  determinations  by  the  Fouque  method 
showed,  in  one  case  andesine,  in  another  labradorite.  It  is  seamed  and  cracked, 
and  the  cracks  are  filled  with  calcite  and  serpentine,  evidently  infiltration  products. 
In  places  the  feldspathic  substance  is  attacked  and  replaced  by  these  minerals. 

Idiomorphic  colorless  augite  is  abundant.  Alteration  to  calcite  and  serpentine 
is  present  in  all  stages,  so  that  while  some  augite  crystals  are  unattacked  others  are 
completely  transformed.  Chlorite  was  not  identified.  Small  apatite  crystals  were 
noted  as  inclusions  in  the  augitc. 

2.  Nearly  fresh   later   andesite  (SJfl)  from    Halifax  shaft,  275  feet  down. — 
Greenish  rock,  showing  phenocrysts  of  glassy  feldspar  (altered  along  the  outside), 
greenish  augite,  and  biotite. 

Under  the  microscope  the  groundmass  is  glassy,  with  fine  microlites  of  fresh 
feldspar  and  augite,  magnetite,  micaceous  hematite,  and  considerable  cloudy  kaolin. 
Quartz  (secondary?)  is  common. 

The  phenocrysts  are  relatively  few.  The  feldspar  is  fresh,  and  one  crystal 
was  determined  as  andesine.  Sometimes  it  is  altered  to  a  cloudy  white  aggregate 
of  kaolin  along  its  margin,  and  in  one  case  a  small  crystal  was  completely  altered  to 
calcite,  kaolin,  and  quartz,  the  clear  quartz  forming  an  envelope  for  the  rest  of  the 
crystal.  The  fresh  feldspar  is  cracked  and  infiltrated  with  micaceous  hematite. 
The  augite  is  pale  green;  no  alteration  of  it  was  noted. 

Fresh  brown  biotite  crystals  sometimes  have  a  border  of  magnetite. 

3.  Entirely  altered  later  aiidesite  (331)  from  North  Star  shaft,  305  feet  down.— 
This   has  a  general   gray  color,  with   dull-white  altered   feldspar  phenocrysts;   it 
contains  many  small  specks  and  seams  of  pyrite.     Under  the  microscope  it  is  seen 
to  be  entirely  altered.     In  the  fine  groundmass  can  be  distinguished  fine  secondary 
quartz  and  chalcedony,  calcite,  pyrite,  siderite,  and  some  zeolite  needles. 

The  phenocrysts  are  also  entirely  altered.  Pseudomorphs  after  biotite  were 
distinguished,  consisting  mainly  of  quartz  and  siderite.  Numberless  tiny  crystals 


240  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 

are  seen  arranged  in  zones  parallel  to  the  rays  of  the  pressure  figure."  These  have 
often  the  characteristic  crystal  form  of  siderite.  They  are  translucent  under  high 
powers,  but  under  lower  powers  show  in  aggregate  the  white,  cloudy  appearance 
characteristic  of  siderite.  Between  these  siderite  zones  is  quartz. 

Pseudomorphs  of  calcite  after  pyroxene,  with  a  few  tin}'  zeolite  needles  and 
some  siderite,  were  noted. 

Pseudomorphs  after  feldspar  consist  of  calcite  and  an  aggregate  of  fibers 
resembling  in  large  part  sericite,  with  some  zeolite  needles. 

Pyrite  and  siderite  are  abundant,  disseminated  or  in  clusters.  The  siderite 
frequently  forms  alteration  rims  around  the  pyrite.  Aggregates  of  siderite  some- 
times show  characteristic  cleavage  and  even  crystal  outline. 

Small  smoky  apatites  occur  in  the  pseudomorphs  after  biotite. 

4-  Entirely  altered  later  andesite  (219)  from  Montana  Tonopah  shaft. — Type 
for  first  278  feet.  Green  pyritiferous  rock,  mottled  with  white  feldspar  pheno- 
crysts,  and  with  apparent  kaolin  coatings  on  joints. 

Under  the  microscope  the  rock  is  seen  to  be  entirely  decomposed.  The  ground- 
mass  is  a  white,  opaque  aggregate  containing  quartz,  some  siderite,  and  much 
cloudy  material  (which  is  very  likely  kaolin),  with  some  chloritic  material. 

The  feldspars  are  completely  altered  to  pseudomorphs,  made  up  of  calcite 
and  a  clear,  colorless  aggregate  showing  sometimes  rather  low  interference  colors, 
while  many  fibers  reach  yellow,  red,  and  even  blue  of  the  first  order.  The 
individual  grains  are  fine,  and  are  often  in  the  shape  of  vermicular  strips,  made  of 
fibers  perpendicular  to  the  long  direction  of  the  strips.  Along  these  strips  the 
extinction  is  wavy,  traveling  from  one  end  to  the  other,  similar  to  the  behavior 
of  spherulites.  Also  occasionally  similar  clear  areas  are  nearly  isotropic,  low, 
doubly  refracting  and  faintly  spherulitic,  like  the  pseudomorphs  after  feldspar 
described  in  specimen  53  (p.  214),  where  the  material  seems  to  be  a  kaolinic  mixture. 
Other  areas  are  of  low-refracting  spherulitic  material,  resembling  chalcedonic 
silica. 

Portions  of  this  white  pseudomorphous  mixture,  showing  still  the  feldspar 
cleavage,  were  separated  from  the  rock,  and  were  tested  chemically  by  Mr.  George 
Steiger,  of  the  United  States  Geological  Survey.  The  calcite  was  leached  out  of 
these  pseudomorphs  and  the  remainder  was  examined  and  found  to  contain,  besides 
considerable  combined  water,  principally  silica  and  alumina,  with  a  small  proportion 
of  magnesia,  roughly  estimated  at  about  4  or  5  per  cent.  The  material  therefore 
appears  to  be  a  mixture  of  an  aluminous  mineral  with  some  magnesian  mineral, 
probably  talc,  and  with  free  silica. 

aSee  Rosenbusch-Iddlngs,  Microscopical  Physiography  of  the  Rock-making  Minerals,  2d.  ed.,  p.  257. 


ALTERATION    OF    THE    LATER    ANDESITE. 


241 


The  optical  characteristics  above  described  indicate  that  the  aluminous  mineral 
is  probably  largely  hydrargillite, a  while  kaolin  is  also  very  likely  present. 

Abundant  pseudomorphs  after  pyroxene  consist  chiefly  of  a  pale  green,  very 
faintly  crystalline  fibrous  aggregate,  which  in  part  seems  to  be  chlorite  and  in 
part  is  certainly  uralitic  hornblende  or  actinolite. 

The  occasional  biotite  crystals  are  bleached  and  contain  secondary  quartz  in 
seams  parallel  with  the  cleavage. 

To  determine  the  character  of  the  carbonates  in  this  rock  they  were  separated 
and  analyzed  qualitatively.  They  were  found  to  consist  of  an  abundance  of 
siderite,  though  the  larger  part  is  calcite.  No  magnesium  carbonate  was  present* 

Analyses  of  described  lypet  of  later  andesite. 
[Nos.  1  and  4  by  Mr.  George  Steiger;  Nos.  2  and  3  by  Dr.  W.  F.  Hillebrand.] 


i. 

2. 

3. 

4. 

SiO2  

57.51 

56  26 

51  64 

43 

A120,  .   .. 

16.55 

16.18 

15  58 

16  49 

Fe203  

3.20 

5  56 

16 

2  86 

FeO  

2  02 

1  17 

58 

6  31 

MgO  

2.30 

2.78 

2  79 

6  19 

CaO  

6  06 

5  07 

6  25 

5  i  -,i| 

Na,O  

2.76 

3.23 

27 

19 

K2O  

2  81 

3  43 

2  46 

84 

H2O-  

1.45 

2.07 

2  56 

3 

H2O+  

2  56 

2  61 

4  43 

7  93 

TiO2  

.80 

.  73 

73 

89 

ZrO.,  

Trace  ? 

Trace  ? 

C02  :  

1.91 

62 

4  24 

4  19 

PA  

30 

32 

31 

36 

SO3  

None. 

None 

03 

08 

Cl  

F 

}  

(0 

(') 

FeS2  

04 

03 

7  89 

2  55 

Cr2O3  

None 

NiO  

Trace 

MnO  

17 

21 

21 

BaO  

12 

(d\ 

07 

SrO  

06 

Trace 

Li2O  

Trace 

m 

100.44 

100.47 

100.13 

100.57 

a  Rosenbusch-Iddings,  Microscopical  Physiography  of  the  Rock-making  Minerals,  3d  ed.,  p.  351. 

6  Determined  by  Mr.  George  Steiger,  of  the  United  States  Geological  Survey. 

c  Not  looked  for. 

d  Not  estimated;  very  little. 

16843— No.  42—05 16 


242 


GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 


DIFFERENCES  OF  COMPOSITION   EXPRESSED  BY  DIAGRAMS. 

The  four  analyses  may  be  represented  by  the  Brogger  diagram  (fig.  75),  in 
the  same  manner  as  employed  for  the  earlier  andesite. 

The    diagrams    show  the   principal    elements   of   fresh    rocks,  and    fulfill   all 


Scale:. 01  (quotient  figure)-^  inch 


KEY 


Flo.  75.— .Diagram  showing  changes  in  composition  during  alteration  of  the  later  andesite. 

ordinary  purposes  for  these,  but  in  altered  rocks  the  altering  agents  have  fre- 
quently entered  into  the  rock  and  constitute  an  important  part  of  its  bulk.  To 
take  cognizance  of  three  of  the  most  important  of  these  agents  in  this  case — water, 


ALTERATION    OF    THE    LATER    ANDESITE. 


243 


carbonic  acid,  and  sulphur  in  the  form  of  iron  sulphide — the  writer  has  constructed 
diagrams  altered  from  the  preceding,  so  that  these  may  also  be  represented  (fig. 
76).  Ten  radii  instead  of  eight  are  taken,  representing  the  different  elements  as 


KEY 


Scale:  .01  (quotient  f  igure)  =  A  inch 
Silican  and  water.OI  *£,  inch 


FIG.  76. — Diagram  showing  changes  in  composition  during  alteration  of  the  later  andesite. 

shown  in  the  key.  The  arrangement  of  the  elements  differs  from  that  in  the 
preceding  diagram,  the  water,  carbonic  acid,  and  iron  sulphide  beiog  grouped 
together,  as  well  as  lime,  magnesia  and  iron,  and  soda  and  potash.  Silica  is 
assigned  one  radius  instead  of  two,  as  in  the  preceding  diagrams,  and  since  its 


244 


GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 


quantity  results  in  an  impracticable  length  for  this  radius,  it  is  represented  on 
half  the  ordinary  scale.  Water  was  so  abundant  in  some  of  the  analyses  that  it 
has  been  represented  in  the  diagrams  on  the  same  scale  as  silica  for  a  similar 
reason.  Only  the  water  given  off  above  106°  C.  has  been  represented,  that  being 
chemically  combined,  while  that  given  off  below  this  point  is  mostly  hygroscopic. 
Otherwise  the  scale  used  is  the  same  as  for  the  preceding  diagram. 

COMPARISON   OF  LATER  ANDESITE  WITH  WASHOE  AND  EUREKA   ROCKS. 

The  first  two  analyses  of  nearly  fresh  rocks  are  similar  to  analyses  of  pyroxene- 
andesites  from  the  Comstock  region  and  from  Eureka,  as  shown  in  the  following 
table.  Nos.  1  and  2  in  the  preceding  table  are  here  called  A  and  B. 

Analyses  of  andesites. 


A. 

B. 

C. 

D. 

E. 

SiO,.. 

57.51 

56  26 

56  71 

56  40 

61  58 

A1,O... 

16.55 

16.  18 

18  36 

15  99 

16  34 

Fe,O, 

3  20 

5  56 

3  26 

FeO 

2.02 

1  17 

6  45 

3  82 

6  42 

MgO               

2.30 

2.78 

3  92 

3  54 

2  85 

CaO  

6.06 

5.07 

6  11 

6  98 

5  13 

Na2O  

2.76 

3.25 

3.52 

3  83 

2  69 

KjO 

2.81 

3  43 

2  38 

1  91 

3  65 

H,O- 

1.45 

2  07 

HO-4- 

9   *>« 

2A1 

1.94 

TiO,  

.80 

.73 

1  14 

68 

CO2  

1.91 

.62 

PA-- 

.30 

32 

32 

28 

FeS,  

.04 

03 

a  64 

«  Loss  on  ignition. 

A.  Mizpah  Extension  shaft,  Tonopah,  Nevada. 

B.  Halifax  shaft,  Tonopah,  Nevada. 

C.  Granular  pyroxene-andesite,  Eldorado,  outcrop,  Washoe,  Nev." 

D.  Pyroxene-andesite,  Sutro  tunnel,  Washoe,  Nev.« 

E.  Pyroxene-andesite,  Richmond  Mountain,  Eureka,  Nev.  * 

DEGREE  OF  ALTERATION  OF  FRESHEST  TONOPAH  LATER  ANDESITE. 

The  freshest  Tonopah  specimens  (A  and  B)  show,  not  only  under  the  microscope 
but  by  the  analyses,  the  beginnings  of  alteration  more  than  do  the  Eureka  and 
Washoe  rocks  with  which  they  are  compared.  The  presence  of  carbonates,  of  a 
greater  amount  of  water,  and  of  a  small  quantity  of  pyrite  indicates  that  the 
former  have  been  somewhat  attacked  by  waters  containing  oxygen,  carbonic  acid, 


a  Hague,  A.,  Mon.  t".  8.  Oeol.  Survey,  vol.  'X.  p.  •»>. 


6  Op.  eit.,  p. '264. 


ALTERATION    OF   THE    LATER    ANDESITE.  245 

and  sulphur,  and  there  has  resulted  partial  hydration,  oxidation,  carbonation,  and 
sulphuration.  The  minerals  developed,  as  shown  by  the  microscopic  description, 
are  serpentine,  siderite,  calcite,  kaolin,  quartz,  hematite,  and  pyrite.  The  consid- 
erable degree  of  oxidation  of  the  iron,  as  compared  with  C  and  E,  is  shown  by 
the  analysis.  There  is  no  evidence,  however,  that  this  incipient  decomposition  has 
been  attended  by  any  change  in  the  relative  amount  of  the  rock  constituents;  it 
was  rather  a  rearrangement  of  the  materials  into  new  minerals  that  were  more 
stable  under  the  new  conditions. 

PRINCIPLES  OF  STUDYING  ALTERATIONS  OF  LATER  ANDESITES. 

No  attempt  has  been  made  to  follow  the  different  stages  of  the  alteration  of 
the  later  andesite  by  analysis,  as  in  the  case  of  the  earlier  andesite,  although  these 
stages  have  been  minutel}7  studied  under  the  microscope.  Therefore,  while  the 
first  two  analyses  (p.  241)  are  of  the  freshest  rocks  obtainable,  the  last  two,  3  and 
4,  are  of  entirely  decomposed  rocks.  In  3  and  4  not  only  has  the  original  mineral 
composition,  as  shown  by  microscopic  examination,  been  completely  obliterated, 
but  in  the  process  there  has  been  an  important  change  in  the  chemical  composi- 
tion of  the  rock  as  a  whole.  This  is  well  illustrated  by  the  diagrams  forming 
figs.  75  and  76. 

It  will  be  noted  that  in  all  four  analyses  the  amount  of  alumina  remains 
practically  constant.  This  oxide  is  perhaps  the  most  refractory  among  rock 
constituents,  and  computations  in  regard  to  loss  or  gain  during  rock  alterations  are 
often  based  on  the  assumption  that  alumina  remains  unaltered.  That  it  probably 
does  not  exactly  do  this,  under  intense  action,  is  shown  by  the  study  of  the  earlier 
andesite  analyses,  where  the  percentages  of  alumina  in  the  bulk  analyses  decrease. 
The  constancy  of  the  alumina  in  the  four  later  andesite  analyses  under  consid- 
eration, however,  is  taken  to  indicate  that  the  alumina  has  not  been  noticeably 
attacked  by  the  alteration,  and  therefore  that  the  comparison  of  the  percentages 
of  the  other  constituents  affords  an  approximately  correct  idea  of  the  loss  and  gain. 


246 


GEOLOGY    OF   TONOPAH    MINING    DISTRICT,  NEVADA. 


ALTERATION   OF   LATER   ANDESITE    FROM    NORTH    STAR   SHAFT. 

To  compare  the  completely  altered  rock  No.  3  with  No.  2  (which  appears  to 
be  the  freshest  of  the  rocks  analyzed,  and  may  he  taken  as  representing  nearly 
the  original  composition  of  No.  3,  except  for  the  partial  oxidation  of  the  iron),  the 
two  analyses  are  given  together  in  the  following  table: 

Analyses  of  later  andesite. 


Rock  No.  2. 

Rock  No.  3. 

SiO2  

56.26 

51.64 

A12O, 

16  18 

15  88 

Fe,O,  . 

5.56 

.16 

FeO 

1  17 

58 

MgO 

2  78 

2  79 

CaO  

5.07 

6  25 

Na2O  

3.25 

.27 

K2O 

3  43 

2  46 

H2O—  

2.07 

2  56 

H2O+  

2.61 

4.43 

TiO2  

.73 

.73 

CO, 

.62 

4  24 

PA  - 

.32 

.31 

SOj  

.03 

FeS, 

03 

7  89 

It  is  noticeable  that  both  analyses  show  the  same  percentages  of  titanium, 
another  highly  refractory  substance,  as  well  as  of  phosphoric  acid.  The  phosphoric 
acid  is  contained  in  the  apatite,  which  resists  decomposition  very  strongly.  This 
strengthens  the  belief  that  these  percentages  afford  a  measure  of  the  change  of 
the  other  constituents. 

Nearly  all  of  the  soda  has  been  extracted,  and  the  silica  has  been  somewhat 
attacked  and  removed.  On  the  other  hand,  the  magnesia  is  unchanged,  as  are 
probably  the  lime  and  potash"  and  the  iron.  The  loss  of  bulk  of  the  rock  occa- 
sioned by  the  removal  of  the  soda  and  silica  is  compensated  by  the  addition  of  large 
quantities  of  carbonic  acid  and  sulphur,  producing  carbonates  of  lime  and  iron  and 
sulphide  of  iron.  It  will  be  noticed  that  most  of  the  remaining  iron  oxide  is  in 
the  ferrous  condition;  this  probably  is  present  as  siderite.  No  dark  iron  or  mag- 
nesian  silicates  were  noted  among  the  decomposition  products.  The  amount  of 
lime  present  is  in  excess  of  the  amount  required  to  form  calcite  with  all  the 
carbonic  acid  in  the  rock;  indeed,  a  small  portion  of  this  carbonic  acid  is  required 

«For  these  conclusions  compare  not  only  the  foregoing  Uible,  but  also  the  table  on  page  219,  showing  variations  ot 
frt»h  rock«  of  this  kind. 


ALTERATION  OF  THE  LATER  ANDESITE. 


247 


to  form  siderite  with  the  ferrous  oxide.  There  remains  a  small  amount  of  lime 
(about  1.35  per  cent)  which  it  is  difficult  to  assign  to  any  of  the  recognized  minerals 
except  the  zeolites,  which  therefore  may  be  supposed  to  be  chiefly  lime  zeolites. 

As  there  are  not  present  any  recognizable  colored  minerals  into  which  the 
magnesia  has  been  transferred  from  its  original  combination  in  the  pyroxene  and 
the  biotite,  the  magnesia  is  probably  contained  in  one  of  the  colorless  minerals, 
and  the  presence  of  talc  in  the  sericitic  aggregate  which  forms  a  large  part  of  the 
feldspar  pseudomorphs  is  indicated,  in  accordance  with  the  conclusions  reached  for 
specimen  No.  4  (see  p.  240).  At  the  same  time  the  analysis  indicates  that  in  this 
aggregate  all  or  a  large  part  of  the  original  potash  in  the  feldspar  is  now  contained 
in  the  form  of  sericite. 

The  sulphur  trioxide  shown  in  the  analysis  of  No.  3  is  probably  contained  in 
gypsum,  a  mineral  abundantly  found  elsewhere  in  this  altered  rock.  It  appears 
to  result  from  the  action  of  waters  containing  sulphuric  acid  (derived  from  oxidation 
of  the  pyrites)  on  the  calcite.  This  is  a  recent  process  and  one  distinct  from  that 
by  which  the  main  alteration  was  produced. 

The  waters  which  produced  this  main  alteration  were,  therefore,  highly  charged 
with  carbonic  acid  and  sulphur;  they  left  these  materials,  with  some  water,  in 
exchange  for  soda  and  silica,  which  they  carried  away. 

ALTERATION    OF   LATER   ANDESITE    FROM   MONTANA   TONOPAH    SHAFT. 

The  relation  which  the  altered  later  andesite  from  the  Montana  Tonopah  shaft 
(No.  4)  bears  to  the  fresh  rock  (No.  2)  may  be  seen  by  comparing  their  respective 

analyses,  which  follow: 

Anali/ses  of  later  andesite. 


Rock  No.  2. 

Rock  No.  4. 

SiO2 

56.26 

43 

Al2Os          

16.18 

16.49 

Fe,O, 

5.56 

2.86 

FeO  

1.17 

6.31 

MgO              

2.78 

6.19 

CaO 

5.07 

5.69 

Na/) 

3.25 

.12 

K,O 

3.43 

.84 

H2O—                                        

2.07 

3 

H,O+. 

2.61 

7.93 

TiO, 

.73 

.89 

COj 

.62 

4.19 

P,(X  . 

.32 

.36 

sos                   

None. 

.08 

FeS2  

.03 

2.55 

248  GEOLOGY   OF   TONOPAH   MINING    DISTRICT,  NEVADA. 

In  No.  2  and  No.  4  again  the  close  correspondence  of  the  alumina,  titanium, 
and  phosphoric  acid — the  last  two  representing  probably,  respectively,  the  resist- 
ant rutile  needles  (sagenite)  in  the  biotite,  and  the  apatite — indicates  that  the 
relative  bulk  of  the  rock  has  not  been  greatly  changed  by  decomposition.  The 
fact,  however,  that  the  percentages  of  each  of  these  constituents  in  No.  4  is 
slightly  in  excess  of  those  in  No.  2  may  be  taken  as  indicating  that  a  slight 
reduction  of  density  has  taken  place. 

Like  rock  No.  3,  rock  No.  4  shows  an  almost  complete  loss  of  soda,  and  a 
similar  loss  of  silica,  both  these  processes  being  carried  further  than  in  No.  3. 
Like  No.  3,  the  lime  has  not  been  noticeably  affected.  Unlike  No.  3,  most  of  the 
potash  has  been  removed,  while  the  iron,  which  in  No.  3  had  not  been  noticeably 
affected,  is  here  present  in  quantity  certainly  largely  exceeding  the  original 
amount.  The  writer  has  computed  the  totul  metallic  iron  present  in  the  different 
rocks  as  follows:  No.  1,  3.82  per  cent;  No.  2,  4.81  per  cent;  No.  3,  4.24  per  cent; 
No.  4,  8.04  per  cent.  The  magnesia,  not  noticeably  affected  in  No.  3,  is  here 
doubled.  Therefore  the  waters  removed  soda,  potash,  and  silica,  and  brought  iron 
and  magnesia  in  partial  compensation,  the  rest  of  the  loss  being  compensated  for 
by  the  addition  of  large  amounts  of  water,  carbonic  acid,  and  sulphur. 

Judging  from  the  microscopic  analysis,  the  iron  of  this  rock  is  chiefly 
contained  in  pyrite,  siderite,  uralite,  and  chlorite;  the  magnesia  in  uralite,  chlorite, 
and  talc.  The  alteration  of  augite  to  chlorite  or  uralite  involves  a  relative  increase 
of  magnesia  and  a  decrease  of  lime.  Dana,  speaking  of  uralite  pseudomorphs 
after  pyroxene,  remarks:" 

"The  most  prominent  change  of  composition  in  passing  from  the  original 
pyroxene  is  that  corresponding  to  the  difference  existing  between  the  two  species 
in  general;  that  is,  an  increase  in  the  magnesium  and  a  decrease  in  the  calcium. 
The  change,  therefore,  is  not  strictly  a  case  of  paramorphism,  though  usually  so 
designated." 

Discussing  the  alteration  of  feldspar  the  same  writer  remarks:* 

"When  the  waters  contain  traces  of  a  magnesian  salt — a  bicarbonate  or  silicate — 
the  magnesia  may  replace  the  lime  or  soda,  and  so  lead  to  a  steatitic  change  or  to  a 
talc  when  the  alumina  is  excluded." 

Dana  indexes  this  "steatitic  mineral"  as  "magnesia  aluminate." 
SIDERITE  AS  AN  ALTERATION  PRODUCT. 

The  abundance  of  siderite  in  the  altered  later  andesite  is  of  some  interest,  as  it 
has  not  been  often  detected  among  the  minerals  resulting  from  hot-spring  action/ 
It  is  almost  always  present  as  a  decomposition  product  of  the  biotite,  pyroxene, 

aSyrtem  of  Mineralogy,  6th  ed.,  p.  890.         l>Op.  cit.,  p.  820.         cLlndgren,  W.,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  SO,  p.  607. 


ALTERATION    OF    THE    LATER    ANDESITE.  249 

magnetite,  etc.,  and  is  nearly  always  closely  associated  with  pyrite.  Usually  the 
two  occur  intercrystallized,  yet  so  clearly  separated  as  to  show  contemporaneous 
crystallization;  sometimes,  however,  a  rim  of  siderite  around  pyrite  indicates  later 
crystallization  for  the  former,  if  not  its  derivation  from  the  pyrite;  while  quite 
as  often  rims  of  pyrite  around  siderite  indicate  a  reversal  of  this  order  of  crys- 
tallization, and  sometimes  the  phenomena  clearly  indicate  that  the  pyrite  has 
formed  at  the  expense  of  the  siderite  (PI.  XXIII).  This  is  in  harmony  with 
the  conclusions  arrived  at  that  the  rock  has  been  altered  by  solutions  at  once 
highly  carbonated  and  sulphureted. 

The  siderite  occurs  usually  as  a  cloudy,  opaque  or  semitranslucent  substance,  of 
a  characteristic  white  color  by  incident  light.  It  has  indeed  usually  the  appearance 
of  the  mysterious  substance  called  leucoxene  by  petrographers,  and  observed  as  the 
decomposition  product  of  ihnenite.  In  many  examples  of  this  mineral  in  the  Tonopah 
andesites,  however,  rhombic  cleavage  has  been  observed,  and  characteristic  rhombic 
crystal  outlines.  The  nature  of  the  mineral  has  also  been  determined  by  chemical 
tests  (p.  241). 

Concerning  similar  siderite  in  the  iron-bearing  rocks  of  the  Mesabi  range  in 
Minnesota,  the  writer  has  made  the  following  statement." 

"  It  is  to  be  noted  that  siderite  *  *  *  surrounds  magnetite  as  a  decomposi- 
tion product,  and  is  cloudy  and  without  crystal  form.  It  thus  comes  under  the 
group  of  decomposition  products  from  magnetite  called  leucoxene.  Rosenbusch 
describes  it  as  an  alteration  product  of  ilmenite,  titaniferous  magnetite,  and  rutile. 
Concerning  its  nature  he  says:6  'Its  chemical  composition  is  not  the  same  in  all 
cases  where  it  has  been  investigated,  and  has  been  considered  the  equivalent  of  a 
variety  of  minerals  (titanite,  anatase,  and  siderite)  by  different  observers.'  In  every 
case  where  this  mineral  is  present  in  these  rocks,  chemical  tests  show  it  to  be 
siderite,  and  no  signs  of  titanium  can  be  found  either  in  it  or  in  the  magnetite 
whence  it  is  derived.  The  existence  of  this  leucoxenic  decomposition  product 
surrounding  magnetite  has  sometimes  been  held  as  sufficient  evidence  that  the 
magnetite  was  titaniferous,  but  it  is  clear  that  it  is  not  necessarily  the  case." 

In  the  altered  "propylitic"  andesite  of  the  Comstock  lode,  which  in  alteration 
resembles  very  nearly  the  later  andesite  of  Tonopah,  Dr.  G.  F.  Becker  suspected 
the  presence  of  siderite.  He  remarks:'" 

"*  *  *  It  seems  certain  that  the  black  border  of  many  hornblendes  has  been 
attacked  and  has  given  place  to  a  transparent  mineral,  which  is  more  or  less  diffused 
in  and  obscured  by  the  groundmass.  The  natural  supposition  is  that  it  is  ferrous 
carbonate." 

aQeol.  Nat.  Hist.  Survey  Minnesota,  Bull.  No.  10,  p.  84. 

b Microscopical  Physiography  of  the  Rock-Making  Minerals,  by  H.  Rosenbusch.    Translated  by  Joseph  P.  Iddings. 
Second,  revised  edition,  p.  165. 

«Mon.  U.  S.  Geol.  Survey,  vol.  3,  p.  215. 


250  GEOLOGY    OK   TONOPAH    MINING    DISTRICT,  NEVADA. 

SCARCITY  OF  EPIDOTE  AS  AN  ALTERATION   PRODUCT. 

Epidote,  so  common  in  similarly  altered  rocks  elsewhere,  is  rare  in  the  later 
andesite  at  Tonopah,  and  where  found  is  often  in  positions  suggesting  that  the 
conditions  of  alteration  ma}-  have  been  abnormal.  For  instance,  bowlders  of  later 
andesite  in  explosive  volcanic  ash  and  breccia  not  far  from  the  contact  of  the  Golden 
Mountain  dacite,  east  of  Mizpah  hill,  show  feldspar  and  biotite  phenocrysts  largely 
altered  to  epidote.  Also  rare  epidote  was  noted  in  one  or  two  specimens  from  the 
Halifax  shaft.  In  a  shaft  sunk  to  "a  depth  of  60  feet  in  decomposed  later  andesite, 
just  west  of  the  Siebert  shaft  dump,  a  specimen  was  collected  which  carried  rather 
abundant  epidote.  This,  however,  is  exceptional,  and  the  typical  alteration  seems 
to  be  illustrated  by  the  detailed  descriptions  and  analyses  given. 

COMPOSITION  OF  ALTERING  WATERS. 

The  waters  which  produced  the  widespread  and  often  profound  alteration  of 
the  later  andesite  were  then,  as  it  seems,  highly  charged  with  carbonic  acid  and 
sulphur  and  contained  magnesia  and  iron.  Since  the3r  did  not  attack  the  lime  in 
the  rocks,  it  is  probable  that  they  contained  also  this  element  in  considerable 
quantity.  In  the  rock  alteration  observed  they  changed  their  composition  chiefly 
by  the  acquirement  of  the  alkalies  and  silica.  They  were  not  ordinary  cool  ground 
waters,  but  clearly  hot-spring  waters.  The  extensive  carbonation  and  sulphura- 
tion  show  this,  as  well  as  the  formation  of  sericite  and  talcose  material,  uralite, 
chlorite,  serpentine,  zeolites,  etc.  Thorough  as  their  work  was,  their  effects  were 
not  so  intense  as  in  the  case  of  the  waters  which  affected  the  earlier  andesite 
in  the  vicinitj'  of  the  veins,  where  the  most  insoluble  elements  were  attacked. 
Moreover,  the  chemical  composition  of  the  waters  was  evidently  quite  different. 

PERIOD  OF  ALTERATION  OF  LATER  ANDESITE. 
ALTERATION    MAINLY   ANTECEDENT   TO    FAULTING. 

The  last  and  most  altered  specimen,  No.  4,  is,  as  already  noted,  the  type  in  the 
Montana  Tonopah  shaft  between  depths  of  90  and  278  feet.  Specimens  taken  at 
various  intervals  show  the  persistence  of  this  general  type  of  alteration  down  to  the 
Mizpah  fault,  which  was  encountered  at  376  feet.  Immediately  beneath  the  fault, 
however,  and  in  the  rest  of  the  workings,  the  earlier  andesite  was  encountered, 
completely  altered  to  the  quartz-sericite  phase.  In  the  Mizpah  mine,  also,  it  was 
noted  that  earlier  andesite  altered  to  quartz  and  sericite  was  separated  sharply  by 
the  Mizpah  fault  from  later  andesite  marked  by  the  strong  development  of  car- 
bonates and  sulphides.  The  indications  are,  therefore,  that  the  faulting  was  not 
only  subsequent  to  the  alteration  of  the  earlier  andesite  (as  is  shown  by  the  fact 
that  it  faults  the  quartz  veins),  but  that  it  was  subsequent  to  the  alteration  of  the 


ALTERATION    OF    THE    LATER    ANDE8ITE.  251 

later  andesite,  which  occurred  at  a  later  period  than  that  of  the  earlier  andesite; 
otherwise  some  trace  or  transition  of  the  later  andesite  alteration  would  be  found 
on  the  earlier  andesite  side  of  the  fault  line. 

RELATION    OF   ALTERATION   TO  VEIN    FORMATION. 

EXUDATION    VEIXLETS    IS    LATER    ANDESITE. 

In  the  later  andesite  occur  many  veinlets  of  calcite,  some  of  gypsum,  and  even 
of  quartz.  They  are  almost  always  very  small  and  nonpersistent,  tilling  cracks, 
and  are  evidently  mainly  the  product  of  lateral  secretion  or  exudation  from  the 
rock.  The  quartz  generally  has  a  chalcedonic  or  jasper}-  look,  as  compared  with 
the  quartz  of  the  earlier  andesite  veins,  although  in  some  cases  the  resemblance 
of  the  two  varieties  of  quartz  to  one  another  may  be  close. 

METALLIFEROUS   VEINS    IN    LATER    ANDESITE. 

Some  larger  veinlets,  probably  of  a  different  origin,  are  composed  of  quartz 
or  quartz  and  calcite,  and  contain  pyrite.  An  assay"  of  such  a  bluish  veinlet  in 
later  andesite,  from  the  east  base  of  Mount  Oddie,  and  near  the  contact  of  the 
Oddie  rhyolite  showed  only  traces  of  gold  and  silver.  It  was  noted  that  these 
veinlets  were  especially  characteristic  of  a  zone  in  the  later  andesite  near  the 
contact  of  the  Oddie  rhyolite. 

Near  the  contact  of  the  glassy  Tonopah  rhyolite-dacite  at  many  points,  as  for 
example,  near  the  Belle  of  Tonopah  shaft,  there  are  numerous  small  veins  of  this 
kind  in  the  intruded  later  andesite.  These  veins  gave  variable  but  generally  small 
assays  for  gold  and  silver,  the  gold  predominating.  In  the  Mizpah  Extension, 
large  veins  of  pyritiferou.s  quartz  were  encountered  in  the  later  andesite,  but 
this  was  at  or  near  the  contact  with  Tonopah  rhyolite-dacite,  which  is,  it  will  be 
remembered,  of  more  recent  date  than  the  later  andesite. 

The  pyrite  in  the  altered  later  andesite  is  sometimes  very  abundant,  and  may 
be  segregated  so  as  to  be  of  striking  appearance,  and  to  suggest  an  ore;  but  assays 
show  in  all  cases  that  the  mineral  is  barren  of  gold  and  silver. 


CONCLUSION. 


It  thus  appears  probable  that  the  more  important  quartz  veinlets  which 
appear  in  the  later  andesite  in  places  were  largely  formed  under  the  influence  of 
solutions  following  the  contacts  of  later  intrusive  rocks — the  rhyolites  and  rhyolite- 
dacites.  This  being  the  case,  it  is  likely  that  a  large  part  of  the  rock  alteration 
just  described  may  have  been  due  to  the  same  causes.  The  entirely  altered 
specimens  3  and  4,  described  and  analyzed,  were  both  near  the  intrusive  contact 
of  the  Oddie  rhyolite,  and  in  general  the  more  altered  portions  appear  to  be  in 

<"  By  R.  H.  Officer  &  Co.,  Salt  Lake  City. 


252 


GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 


the  vicinity  of  the  large  subsequent  igneous  intrusions.  It  is  therefore  likely 
that  the  alteration  of  the  later  andesite  was  largely  produced  by  waters  which 
followed  later,  chiefly  rhyolitic,"  intrusions  into  it. 

ALTERATION  OF  THE  ODDIE  RIIYOLITE. 

Some  partial  anah'ses  were  made,  to  show  the  composition  of  the  fresh  and  the 
altered  white  Oddie  rhyolite.  As  a  rule  this  rock  is  quite  fresh,  even  when  close  to 
the  intensely  altered  earlier  and  later  andesites.  Sometimes,  however,  especially 
along  faults  and  watercourses,  the  rhyolite  disintegrates  and  the  feldspar  is  partly  dis- 
solved out,  leaving  cavities,  while  the  scant  biotite  of  the  fresh  rock  has  disappeared. 

The  partial  analyses  are  as  follows: 

Analyses  of  Oddie  rhyolite. 
[By  Dr.  E.  T.  Allen.] 


1  (376). 

2(837). 

3(227). 

SiO2 

75.66 

76.57 

77  71 

CaO  

.47 

Na^O 

1.70 

96 

17 

K2O 

4.94 

5  81 

4  04 

The  first  two  analyses  being  of  fresh  rock,  the  difference  in  the  chemical 
composition  is  probably  original.  This  difference  was,  indeed,  noted  in  the  field, 
where  the  rhyolite  of  Rushton  Hill  (No.  1)  was  observed  to  have  a  slightly  more 
basic  aspect  than  the  rhyolite  of  Mount  Oddie  (No.  2),  and  to  approach  in 
appearance  the  siliceous  dacite  of  Golden  Mountain  near  by.  No.  3,  however,  is 
Oddie  rhyolite  which  was  probably  originally  of  a  composition  similar  to  No.  2, 
and  the  chemical  change  undergone  on  alteration  seems  to  have  been  a  slight 
increase  in  silica  and  a  loss  of  the  alkalies,  especially  soda. 

The  microscopic  description  of  No.  3  is  as  follows: 

3.  (Specimen  227)  Mizpah  Extension  shaft,  385  feet  down.  Hand  specimen  is 
white  and  hard,  but  shows  cavities  due  to  the  dissolution  of  feldspar  phenocrysts. 
There  is  no  biotite.  Under  the  microscope  there  are  also  no  signs  of  biotite,  and 
the  feldspars  are  entirely  altered  to  a  sericite  aggregate,  both  in  the  phenocrysts 
and  in  the  groundmass.  The  phenocrysts  consist  of  abundant  quartz,  with  sericite 
areas  representing  original  feldspars,  while  the  groundmass  consists  of  an  aggregate 
of  crystalline  granular  quartz,  much  coarser  than  in  the  fresh  rock  and  sericite. 
The  size  of  the  quartz  grains  in  the  groundmass  is  evidently  due  to  enlargement  by 
the  waters  which  produced  the  alteration,  for  crystal  faces  are  frequent  and  such 
idiomorphic  grains  frequently  impinge  upon  the  area  of  the  original  idiomorphic 
feldspar  phenocrysts,  now  altered  to  sericite. 

•'  'l  in'  glassy  Tonopah  rhyollKMlaclte  Is  a  rhyolitic  variety  (p.  69.) 


CHAPTER    VII. 

ORIGIN  OF  MINERAL  VEINS. 

ORIGIX  OF  THE  MIXERALJZIXG   AXD  ALTERING  WATERS. 

ANTITHESIS    BETWEEN    WATERS    AND    ASSOCIATED    ROCK. 

In  view  of  the  composition  of  the  waters  which  produced  the  veins  and  the 
chief  alteration  of  the  early  andesite,  it  has  been  argued  that  they  were  rich  in 
silica  and  potash  and  noticeably  poor  in  the  other  common  rock-forming  elements. 
They  seem  to  have  directly  followed  the  earlier  andesite  eruption.  In  considering 
the  alteration  of  the  later  andesite  in  the  vicinity  of  Mount  Oddie,  it  has  been 
concluded  that  the  waters  which  wrought  the  change  were  rich  in  magnesia,  lime, 
and  iron,  and  low  in  silica  and  the  alkalies;  in  this  case  the  data  seem  to  point 
to  the  explanation  that  the  waters  followed  the  eruption  of  the  Oddie  rhyolite. 
Both  are  concluded  to  have  been  hot-spring  waters,  which  were  active  after  volcanic 
eruptions  for  a  i-elatively  short  time,  geologically  speaking,  and  which  differed  in 
composition  as  much  as  the  rocks.  If  these  conclusions  are  true,  it  is  right  to 
notice  an  apparent  antithesis  in  each  case  between  the  composition  of  the  erupted 
rock  and  that  of  the  accompanying  and  succeeding  hot  solutions.  The  eruption 
of  the  earlier  andesite,  a  rock  of  intermediate  composition,  containing  perhaps 
about  60  per  cent  of  silica,  and  about  five  times  as  much  soda,  lime,  iron,  and 
magnesia  as  it  does  potash,  was  followed  by  the  advent  of  waters  which  were 
rich  in  the  elements  characteristic  of  extremely  acid  rocks  (alaskites) — namely, 
silica  and  potash — with  the  proportion  of  silica  probably  largely  in  excess  of  that 
in  these  rocks  and  probably  approximating  that  in  feldspathic  quartz  veins  of 
granitic  origin,  as  the  composition  of  the  Tonopah  veins  indicates.  The  eruption 
of  the  Oddie  rhyolite,  a  rock  made  up  almost  entirely  of  silica  and  potash,  with 
alumina,  and  only  trifling  quantities  of  magnesia,  lime,  and  iron,  was  followed  by 
the  advent  of  waters  rich  in  these  three  last-named  elements  (which  are  charac- 
teristic of  basic  rocks)  and  poor  in  the  elements  represented  in  the  rhyolite  itself. 

Testing  this  latter  conclusion,  we  may  recall  the  calcitic  veins  of  Ararat 
Mountain,  which  are  certainly  directly  due  to  hot  solutions  that  ascended  immedi- 
ately after  the  eruption  of  the  neck  or  plug  of  Oddie  rhyolite  (p.  101).  It  has  been 
shown  that  these  waters  give  evidence  of  having  contained  chiefly  lime,  iron, 

253 


254  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 

manganese,  and  silica.  They  have  produced  silicitication,  and  have  deposited  silica 
in  fissures,  but  the  silica  is  usually  greatly  exceeded  by  the  calcite  (figs.  14,  15). 
These  waters  then  were  also  characterized  by  the  materials  of  basic  rather  than  of 
acidic  igneous  rocks. 

Along  the  contact  of  the  dacitic  rocks  there  has  frequently  been  profound 
alteration  of  the  later  andesite,  but  the  process  has  not  been  studied  sufficiently  to 
give  definite  conclusions.  A  specimen  from  the  later  andesite  near  the  Molly  shaft, 
at  the  contact  with  the  Golden  Mountain  dacite,  is  entirely  altered  to  calcite  and 
quartz,  the  former  unusually  abundant,  with  siderite  and  pyrite,  etc.  At  the  Belle 
of  Tonopah  shaft  specimens  of  the  later  andesite  near  the  contact  with  the  glassy 
Tonopah  rhyolite-dacite  are  largely  altered  to  calcite,  together  with  quartz  and 
probable  sericite;  other  specimens  near  here  are  more  plainly  silicified,  but  are 
ferruginous.  The  glassy  rhyolite-dacite  itself,  near  the  contact,  is  often  silicified, 
but  shows  frequently  considerable  epidote.  Calcification  as  well  as  silicitication  is 
therefore  suggested  in  all  these  instances. 

Omitting,  therefore,  as  without  sufficient  data,  the  consideration  of  the  solu- 
tions accompanying  the  rhyolite-dacites  and  referring  only  to  the  Oddie  rhyolite 
and  the  earlier  andesite,  the  conclusions,  if  correct,  may  have  a  bearing  on  the 
source  of  these  solutions. 

THEORY  OF  ATMOSPHERIC  ORIGIN  OF  HOT  SPRINGS. 

There  are  two  possible  explanations  of  hot  springs  in  general.  One  is  that 
atmospheric  water,  of  which  such  a  large  quantity  sinks  below  the  surface, 
becomes  warmer  in  depth  by  the  natural  increment  of  temperature  or  in  volcanic 
regions  by  the  residual  heat  of  the  rocks,  and  on  finding  a  channel  ascends  toward 
the  surface  as  hot  water,  carrying  with  it  materials  which  it  has  dissolved  out  of 
the  rocks  on  its  passage.  A  physical  objection  to  this  theory  is  that  surface 
water  could  hardly  work  its  way  downward  against  pressure,  to  the  depths  neces- 
sary to  become  highly  heated.  This  has  been  met  by  the  experiment  of  Daubre'e, 
which  showed  that  water  would  work  itself  downward  through  a  solid  marble 
slab  by  capillarity,  in  spite  of  the  resistance  offered  by  a  strong  pressure  on  the 
underside  of  the  slab.  It  has  been  argued  that  by  such  capillary  circulation 
the  supplies  of  hot  springs  may  be  replenished. 

THEORY  OF  MAGMATIC  ORIGIN  OF  HOT  SPRINGS. 

The  other  explanation  goes  back  to  the  hypothetical  origin  of  the  atmos- 
pheric or  surface  water  at  the  period  of  the  consolidation  of  the  globe.  Accord- 
ing to  the  commonly  accepted  theory,  when  the  molten  or  fluid  earth  stuff  cooled 
and  was  consolidated,  a  large  part  of  the  contained  water  was  separated,  and  by 
reason  of  its  great  mobility  formed  the  oceans.  Processes  similar  to  those  which 


MAGMATIC    ACTION    OF    HOT    SPRINGS.  255 

thus  went  on  on  a  large  scale  in  primeval  times,  it  is  argued,  still  go  on  when- 
ever a  body  of  magma  consolidates;  a  large  part  of  the  water  of  this  fluid  material 
is  separated  and  expelled  and  most  of  it  escapes  to  the  surface  as  hot  springs, 
adding  to  the  surface  waters  already  originated  by  similar  separations. 

Of  these  two  explanations,  the  former  may  seem  more  familiar  and  probable, 
because  of  our  acquaintance  with  ordinary  surface  waters  and  our  lack  of  intimacy 
with  newborn  magmatic  waters.  Yet  the  magmatic  explanation  is  the  only  one 
of  whose  possibility  we  have  ocular  demonstration.  We  have  no  such  demon- 
stration that  surface  waters  can  penetrate  downward  till  they  are  heated  far 
above  the  boiling  point  and  then  rise  again  and  emerge,  and  we  can  reach  such  an 
idea  only  by  a  process  of  speculation  which  is  not  even  logical  reasoning.  On  the 
other  hand,  the  vast  quantities  of  water  vapor  given  off  by  lavas  at  many  volcanic 
centers  afford  proof  that  water  is  present  in  these  unconsolidated  magmas  and 
separates  on  cooling.  Furthermore,  the  phenomena  of  contact  metamorphism, 
especially  that  connected  with  siliceous  rocks,  show,  as  has  often  been  pointed  out, 
that  in  depth  similar  water  vapor  is  expelled  from  cooling  rock,  even  under  great 
pressure. 

Volcanic  activity  has  sometimes  been  ascribed  to  the  infiltration  of  surface 
water,  which,  on  coming  into  contact  with  heated  rocks  below,  causes  explosions 
and  extravasations  of  lava;  and  the  water  given  off  from  the  cooling  lavas  is  thus 
thought  to  have  a  surface  origin.  Many  facts,  however,  which  can  not  be  gone  into 
here  are  against  this  hypothesis.  Concerning  the  steam  given  off  at  Vesuvius, 
Prof.  E.  Suess  remarks: a 

u*  *  *  j£  js  a(.  jeast;  certain  that  the  quantities  of  steam  issuing  from  the 
parasitic  crater  must  have  come  from  a  zone  in  which  the  temperature  equals  or 
exceeds  the  melting  point  of  most  rocks,  and  in  which  there  can  be  no  question  of 
porous  or  f ragmen tal  rocks,  and  therefore  no  question  of  infiltration  of  vadose* 
water." 

That  is,  the  principle  of  capillarity  above  referred  to  can  not  apply  to  rocks  at 
these  great  temperatures  and  can  not  explain  the  water  in  lavas. 

When  the  upward  movements  in  the  lava  bodies  have  ceased  and  a  crust  of 
cooled  and  solid  rock  has  congealed  at  the  surface,  consolidation  will  progress 
downward.  The  aqueous  vapor  given  off  from  this  lower  cooling  lava  will  become 
condensed  to  water  on  its  passage  through  the  cooled  crust  and  will  so  emerge. 
It  seems,  therefore,  impossible  to  escape  the  conclusion  that  at  least  some  hot 
springs,  the  after-phenomena  of  volcanic  activity,  have  the  origin  above  described, 
and  contain  newborn  water  separated  from  the  magma.6' 

a«eog.  Jour.,  vol.  20,  p.  519. 

6  Surface. 

eSuch  water  has  been  called  juvenile  or  primitive  by  Professor  Suess,  and  hifpofjene  by  one  of  his  translators,  to 
distinguish  it  from  the  shallow  underground  water  derived  irom  the  surface,  or  radose  water,  the  latter  term  having 
been  proposed  by  Posepny  in  his  essay  on  ore  deposits. 


256  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 

Vadose  or  surface-derived  descending  water  must  meet  and  mingle  with  these 
escaping  magmatic  waters,  must  change  their  composition  and  mitigate  their  heat, 
and  the  mingled  waters  must  in  many  cases  emerge  on  the  surface  as  warm 
springs. 

CHARACTERISTICS    OF    THE    NEVADA    SPRINGS. 

The  conception  that  the  hot  springs  of  the  volcanic  region  of  Nevada  were 
largely  supplied  by  magmatic  or  primitive  water  from  the  cooling  subterranean 
lava  was  formed  by  the  writer  in  the  field  in  1902,  before  reading  Professor  Suess's 
paper,  above  referred  to. 

On  account  of  the  exceeding  aridity  of  the  Great  Basin,  there  are,  as  a  rule, 
no  flowing  surface  waters,  the  whole  supply  emerging  from  the  ground  as  springs. 
These  springs  are  hot,  warm,  or  cold.  The  cold  springs  usually  emerge  from 
depressions,  fault  or  fracture  lines,  and  are  especially  found  near  the  base  of  the 
desert  mountain  ranges.  They  usually  show  two  characteristics  which  indicate  that 
they  are  of  vadose  origin:  (1)  They  fluctuate  with  the  season,  being  abundant  in 
the  spring  and  often  becoming  scanty  or  dry  at  the  close  of  the  summer,  and  (2) 
they  become  more  numerous  and  copious  in  the  regions  of  greater  precipitation 
and  very  rare  in  the  more  arid  portions.  Near  the  Sierra  Nevada  and  in  the 
region  just  east,  which  receives  the  overdrift  from  the  Sierra  precipitation  in  the 
shape  of  relatively  abundant  snows  and  more  frequent  rains,  the  cold  springs 
emerging  from  the  base  of  the  mountains  are  numerous  and  so  large  as  to 
frequently  form  short  streams,  sufficing  often  for  agriculture,  and  producing  a 
fringe  of  ranches  along  the  mountain  base,  such  as  that  which  borders  the  eastern 
base  of  the  White  Mountains  in  Fish  Lake  Valley.  The  hot  springs,  on  the 
other  hand,  so  far  as  the  writer's  experience  and  information  go,  do  not  show 
these  characteristics  of  vadose  origin;  they  show  no  change  with  the  season  and 
are  not  noticeably  associated  with  regions  of  greater  precipitation.  They  are 
noticeably  associated  with  areas  of  volcanic  rocks  and  are  scattered  all  over  these 
areas,  being  often  very  vigorous  in  the  heart  of  an  arid  region  and  sometimes 
sufficiently  copious  to  form  short  streams. 

COUPLING  OF  HOT  AND  COLD  SPRINGS. 

It  is  a  matter  of  frequent  remark  in  this  dry  Nevada  region  that  hot  springs 
and  cold  springs  are  frequently  coupled  together  and  emerge  within  a  short 
distance  of  each  other.  The  writer  has  observed  an  instance  of  this  at  the  village 
of  Silver  Peak,  25  miles  southwest  of  Tonopah,  where  a  spring  of  nearly  scalding 
temperature  and  one  at  most  lukewarm  or  tepid  emerge  from  the  edge  of  the 
desert  plain  at  the  east  base  of  the  Silver  Peak  Range  within  a  score  of  feet  of 
each  other.  These  are  evidently  waters  from  diflerent  sources,  and  their  coupling 


CHARACTERISTICS    OF    NEVADA    SPRINGS.  257 

must  be  ascribed  to  their  having  neighboring  and  probably  parallel  channels  along 
the  same  fracture  zone.  Decomposed  rock  along  such  a  fracture  zone  would 
form  an  effective  barrier,  preventing  currents  from  mingling  the  waters  and 
averaging  their  temperatures.  The  cool  water  is  evidently  vadose,  and  probably 
represents  a  part  of  the  atmospheric  waters  which  fall  upon  the  Silver  Peak 
Range,  while  the  hot  waters  have  a  distinct  and  vastly  deeper  origin.  It  is  clear, 
however,  that  in  many  similar  cases  the  two  currents  of  water  must  mingle, 
appearing  at  the  surface  as  springs  of  varying  warmth  and  of  composite  origin. 
In  seeking  to  understand  the  nature  of  the  Silver  Peak  hot  springs  the  writer 
learned  from  the  inhabitants  of  the  village  a  significant  fact.  According  to  them 
the  water  of  the  hot  springs  is  much  hotter  in  winter  and  fall  than  in  summer 
and  spring,  so  that  in  the  former  seasons  much  more  cold  water  must  be  added 
to  bring  it  down  to  a  temperature  requisite  for  bathing.  This  indicates  that  the 
temperature  of  the  hot  water  is  really  modified  by  the  cool  vadose  water,  the 
modifying  being  characteristic  of  the  seasons  when  the  melting  of  the  snows 
provides  a  considerable  supply  to  the  shallow  underground  circulation. 

THE    DEVILS    PUNCHBOWL. 

Mr.  J.  L.  Butler,  the  discoverer  of  Tonopah  and  an  old  inhabitant  of  the 
region,  has  described  to  the  writer  a  hot  spring  in  Monitor  Valley,  not  far  from 
Belmont,  which  is  45  miles  northeast  of  Tonopah.  This  spring  occupies  a  cup- 
shaped  depression — probably  formed  by  sinter  accumulations — known  as  the  Devils 
Punchbowl.  This  depression  is  reported  to  be  30  feet  in  diameter  and  to  be  full 
of  hot  water  up  to  a  point  30  feet  below  the  top.  The  level  of  the  water  has 
gone  down  3  feet  in  thirty  years  and  the  water  has  become  cooler.  Formerly 
more  gas  than  at  present  was  emitted,  and  occasional  flames  were  seen.  This 
change  is  apparently  a  secular  one,  strikingly  different  from  the  seasonal  variations 
of  vadose  springs,  and  suggesting  as  a  cause  the  diminution  of  volcanic  energy 
in  this  region  of  abundant  Tertiary  volcanics. 

AMOUNT    OF    PRESENT    AND    RECENT    HOT-SPRING    ACTION. 

Similar  hot  springs,  some  of  them  boiling,  abound  in  the  region  and  surround 
Tonopah  on  all  sides.  Volcanic  activity  has  been  great  in  this  province  for  a 
prolonged  period,  lasting  from  the  beginning  of  the  Tertiary  to  within  a  few 
hundred  years  ago.  At  Silver  Peak  is  a  small,  undefaced  basalt  crater,  which  is 
younger  than  the  detritus  of  the  valley,  and  can  hardly  be  more  than  a  few 
hundred  years  old;  and  there  are  a  number  of  other  craters,  such  as  those  in  and 
near  Lake  Mono — described  by  Russell — which  are  comparatively  recent.  That 
many  of  the  hot  springs  which  accompanied  or  followed  the  different  manifestations 
16843— No.  42—05 17 


258  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 

of  volcanic  activity  are  now  extinct  is  shown  by  the  characteristic  effects  of  these 
springs  in  many  localities,  indicating  that  the  number  of  such  springs  was  probably 
formerly  greater  than  at  present. 

ORIGIN    OF    EXTINCT    HOT    SPRINGS    AT    TONOPAH. 

CONNECTION    WITH    VOLCANIC    ERUPTIONS. 

At  Tonopah  evidence  has  been  given  to  show  that  after  several  of  the  volcanic 
eruptions  waters  ascended,  altered  the  rocks,  deposited  new  and  removed  old 
material,  and  became  extinct  in  a  relatively  short  space  of  geologic  time.  If  the 
reasoning  given  in  the  preceding  pages  is  correct,  it  is  very  difficult  to  explain 
the  totally  different  composition  reasoned  out  for  the  waters  at  different  periods 
on  the  Itypothesis  that  the  mineralizing  waters  were  of  atmospheric  origin  and 
derived  their  material  from  solution  of  the  rocks  which  they  traversed.  These 
ascending  waters  followed  channels  practically  side  by  side,  if  not  in  many  cases 
nearly  the  same,  and  it  is  most  natural  to  suppose  that  the  rocks  which  they 
traversed  were  not  greatly  different. 

CONSEQUENCES  OF  ANTITHESIS  BETWEEN    ROCKS  AND  WATERS. 

A  second  important  consideration  is  the  apparent  antithesis  pointed  out 
between  the  contents  of  waters  at  different  periods  and  the  composition  of  the 
lavas  which  they  followed."  There  is  indeed  apparently  a  relation,  but  it  is  the 
opposite  one  from  what  would  result  had  the  waters  derived  their  mineral 
contents  from  the  leaching  of  these  lavas  by  ordinary  atmospheric  waters.  The 
same  difficulty  presented  itself  to  Professor  Suess  and  many  other  investigators 
in  considering  the  origin  of  the  Carlsbad  Springs  in  Germany.*  The  amount  of 
soda  and  lime  in  these  springs  suggests  that  the  bulk  of  the  matter  in  solution 
must  be  derived,  not  from  the  granite  of  the  country,  but  from  some  unknown 
source.  The  quantity  of  the  water  and  the  carbonic  acid  at  Carlsbad  were  also 
inexplicable  on  the  hypothesis  that  the  waters  were  of  meteoric  origin,  and  led 
Professor  Suess  and  others  to  believe  that  the  waters  and  their  contents  were  of 
magmatic  origin. 

MEANING  OF  NATURE  OF  METALS  IN  VEINS. 

A  third  consideration  is  the  peculiar  combination  of  materials  in  the  waters 
which  produced  the  veins  in  the  earlier  andesite.  Not  only  is  the  abundance  of 
silica  and  potash,  together  with  the  lack  of  sodium,  magnesium,  lime,  iron,  etc.— 
elements  more  characteristic  of  the  andesite- — difficult  of  explanation  on  the  theory 
of  leaching  from  the  traversed  rocks,  but  also  the  presence  of  unusually  large 

aThe  writer  has  at  present  no  explanation  of  this  antithesis  to  offer. 
OSuesi,  E.,  Geog.  Jour.,  vol.  20,  p.  617. 


ORIGIN   OF   HOT   SPRINGS.  259 

quantities  of  the  rare  metals  silver  and  gold,  and  unusually  small  ones  of  the 
commoner  ones  copper,  lead,  and  zinc.0  The  amount  of  silver  by  weight  in  these 
primary  ores,  so  far  as  they  have  been  developed,  seems  to  exceed  that  of 
either  of  the  three  last-named  metals.  No  such  results  as  this  could  be  expected 
were  the  metals  derived  from  leaching  of  the  andesite.  Plainly  some  process  of 
separation  and  concentration  has  furnished  the  noble  metals  contained  in  the 
mineralizing  waters,  separating  them  from  the  baser  metals.  Nickel  is  present  in 
the  fresh  later  andesite  (p.  34)  and  was  detected  in  the  fresh  earlier  andesite  of 
Eureka;  yet  this  metal  has  not  been  detected  in  the  ores  in  either  camp.  In  the 
rocks  near  the  Comstock  lode  analyses  conducted  by  Dr.  G.  F.  Becker*  showed 
small  quantities  of  silver  and  gold,  whence  it  was  concluded  that  the  ores  of  the 
lode  had  been  derived  from  the  wall  rocks  (by  lateral  secretion).  But  later 
investigations  on  the  subject  of  the  presence  of  the  precious  metals  in  rocks 
show  that  these  metals  are  very  frequently  present  in  rocks  not  associated  with 
ore  deposits,  as  well  as  in  those  that  are;0  and  the  results  of  the  assays  tabulated 
by  Becker  do  not,  to  the  writer's  mind,  indicate  any  connection  between  these 
traces  of  metals  and  the  ores  of  the  Comstock  lode.  At  Washoe,  as  at  Tonopah, 
the  theory  of  leaching  from  wall  rocks,  or  lateral  secretion,  indeed,  leaves 
unexplained  the  presence  of  silver  and  gold  in  such  large  quantities,  relatively 
to  the  commoner  metals.  The  view  concerning  this  problem  at  the  Comstock, 
expressed  by  von  Richthofen,''  seems  to  the  writer  especially  illuminating,  and 
applicable,  as  well,  to  the  similar  situation  at  Tonopah.  Von  Richthofen  remarks: 

"  We  have  in  the  elements  evolved  during  the  first  two  periods  of  solfataras — 
namely,  fluorine,  chlorine,  and  sulphur — all  the  conditions  required  for  filling  the 
Comstock  fissure  with  such  substances  as  those  of  which  the  vein  is  composed. 
Steam,  ascending  with  vapors  of  fluosilicic  acid,  created  in  its  upper  parts  (by 
diminution  of  pressure  and  temperature,  according  to  well-known  chemical  agencies) 
silica  and  silicofluohydric  acid,  the  former  in  solid  form,  the  latter  as  a  volatile  gas, 
which  acts  most  powerfully  in  decomposing  the  rocks  it  meets  on  its  course.  The 
chloride  of  silicon  in  combination  with  steam  forms  silica  and  chlorhydric  acid. 
Fluorine  and  chlorine  are  the  most  powerful  volatilizers  known,  and  form  volatile 
combinations  with  almost  every  substance.  Besides  silicon,  the  metals  have  a  great 
affinity  with  them.  All  those  which  occur  in  the  Comstock  vein  could  ascend  in  a 
gaseous  state  in  combination  with  one  or  the  other  of  them.  * 

"It  is  a  fact  worthy  of  notice  that  there  is  scarcely  a  single  chemical  agent, 
excepting  fluorine  and  chlorine,  which  would  not  carry  metallic  substances  into 

a  Since  the  above  was  written  the  important  discovery  of  the  presence  of  selenides  has  been  made  by  Doctor  Hillebrand. 
See  pp.  89,  90.  Doctor  Hillebrand  remarks  that  the  presence  of  selenides,  and  the  absence  of  their  closely  associated 
element,  tellurium,  indicated  some  unusual  process  of  separation.  Tellurides  have  been  found  at  Goldfield,  28  miles 
south  of  Tonopah,  also  in  Tertiary  volcanic  rocks;  and  from  that  camp  selenides  have  not  yet  been  reported. 

6Mon.  U.  8.  Geol.  Survey,  vol.  3,  pp.  184,  155,  223. 

c  As  examples,  see  Wagoner,  Luther,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  31,  pp.  798-810. 

dMon.  U.  S.  Geol.  Survey,  vol.  3,  pp.  19,  20. 


260  GEOLOGY    OF   TONOPAH   MINING   DISTRICT,   NEVADA. 

fissures  in  exactly  or  nearly  the  reverse  quantitative  proportion  from  that  in  which 
they  occur  in  silver  veins.  Iron  and  manganese  are  not  only  more  abundant  in 
rocks,  but  also  much  more  easily  attacked  and  carried  away  by  acids,  than  silver 
and  gold.  The  proportion  of  these  to  the  former  ought,  therefore,  to  be  still 
smaller  in  mineral  veins  than  it  is  in  rocks,  and  lead  and  copper  ought  to  be  more 
subordinate,  if  their  removal  from  their  primitive  place  had  been  effected  by  other 
agents  than  fluorine  and  chlorine.  Only  these  two  will  first  combine  with  those 
metals  which  are  most  scarce  in  rocks  and  relatively  most  abundant  in  silver  veins, 
and  they  are  probably  the  only  elements  which  have  originally  collected  them 
together  into  larger  deposits,  though  these  may  subsequently  have  undergone 
considerable  changes,  and  water  may  have  played  altogether  the  most  prominent 
part  in  bringing  them  into  their  present  shape." 

NATURE  OF  SOLFATARIC  ACTION. 

Concerning  the  nature  of  solfataras,  the  following  extracts  are  quoted  from 
Professor  Bonney's  Volcanoes  (p.  52): 

"In  the  intervals  between  the  paroxysmal  phases  most  volcanoes  emit  simply 
steam,  and  all  in  their  decadence  pass  through  a  longer  or  shorter  period  when  it 
alone  is  ejected.  This  is  often  termed  the  solfatara  stage,  from  the  crater  of  that 
name  in  the  Phlegrsean  Fields.  Like  most  of  those  in  this  district,  the  cone  is  low 
and  the  crater  wide;  the  floor  is  a  level,  sometimes  marshy,  plain,  surrounded  by 
steep  walls  of  ashy  materials,  perhaps  a  hundred  feet  in  height.  The  last  eruption 
was  in  1189,  when  a  stream  of  trachytic  lava  was  discharged  from  the  southern  side 
of  the  crater;  but  now  the  sole  sign  of  activit}',  except  some  boiling  puddles  in  one 
part  of  the  floor,  is  to  be  found  at  the  foot  of  the  crag  on  the  side.  Here,  from  a 
fissure  in  the  inclosing  wall,  something  like  the  adit  of  a  mine,  a  column  of  steam  is 
ejected  to  a  height  of  6  or  7  yards.  The  steam  commonly  is  more  than  the 
vapor  of  water.  Such  acids  as  hydrochloric  or  sulphuric  are  often  present;"  that 
of  the  solfatara,  as  we  can  see  from  the  sulphur  abundantly  deposited  round  the 
aperture  and  the  rotten  condition  of  the  adjacent  rocks,  is  no  exception  to  the  rule. 
No  doubt  the  materials  in  and  about  a  vent  must  undergo  considerable  chemical 
changes  when  the  volcano  is  passing  through  this  stage  in  its  history." 

Professor  Bonney  finishes  his  summary  of  the  description  of  volcanic  eruptions 
as  follows  (p.  62): 

"An  eruption  is  generally  ushered  in  by  earthquake  shocks,  is  always  associated 
with  explosions,  and  is  frequently  concluded  by  the  emission  of  a  considerable  mass 
of  lava.  Great  quantities  of  water  are  discharged  in  the  form  of  steam,  and  the 
phenomena  of  an  eruption  are  closely  imitated  by  geysers.  Other  vapors  also  are 
discharged,  and  the  solfatara  stage  of  a  dying  volcano  commonly  ends  with  the 
exhalation  of  carbonic  acid  or  some  such  gas;  perhaps  the  last  stage  of  all  may  even 
be  a  cold  mineral  spring." 

a  The  steam  emitted  from  Vesuvius  in  January,  1876,  was  acid  with  these,  particularly  the  former.    Steel  was  rusted 
and  clothes  were  slightly  altered  in  color  in  the  course  of  an  hour  or  two. 


GENESIS    OF   TONOPAH    ORES.  261 

Professor  Suess,  in  the  essay  referred  to,"  thus  describes  the  funiarolic  activity 
at  Vesuvius: 

"Turning  now  to  the  gases  accompan3ring  the  eruptions.  After  steam,  chlorine 
and  gases  containing  sulphur  are  the  most  important,  and  carbonic  acid  gas  comes 
next.  Their  occurrence  follows  a  definite  law.  So  far  as  it  has  been  possible  to 
approach  them,  all  fumaroles  actually  within  vents  contain  steam;  but  the  hottest 
fumaroles  (over  500°  C.)  on  the  surface  of  cooling  lava  streams,  where  approach  is 
easier,  are  dry.  In  the  emanations  from  these  high-temperature  fumaroles  are  found 
chlorine  compounds,  and  along  with  them  fluorine,  boron,  and  phosphorus — sub- 
stances which  are  the  first  to  disappear  as  the  temperature  of  the  f  umarole  sinks. 
Sulphur  persists  longer,  often  combined  with  arsenic.  Carbonic  acid'  is  given  off 
freely  till  a  much  later  stage,  sometimes  till  the  fumarole  is  comparatively  cool, 
notwithstanding  that  it  is  observed  in  the  hottest  dry  fumaroles.  Fumaroles  in 
different  'phases  of  emanation'  may  occur  quite  near  one  another.  The  steam  of  the 
volcano  can  not  be  derived  from  vadous  infiltration,  for  if  it  is,  whence  the  carbonic 
acid  ?  Both  must  come  from  the  deeper  regions  of  the  earth.  They  are  the  outward 
sign  of  the  process  of  giving  off  gases  which  began  when  the  earth  first  solidified, 
and  which  to-day,  although  restricted  to  certain  points  and  lines,  has  not  yet  come  to 
a  final  end." 

MINERALS   DEPOSITED  AROUND  FUMAROLES. 

Around  the  orifices  of  the  steam  jets  (fumaroles)  at  Vesuvius  sulphides  of  arsenic 
and  mercury  and  chlorides  of  copper  and  lead  have  been  deposited,  showing  the 
efficacy  of  such  gases  in  separating,  dissolving,  and  precipitating  these  relatively 
rare  substances.  Dana*  quotes  Mallet  as  authorit\r  for  the  statement  that  native 
silver  ore  occurs  rarely  in  volcanic  ashes. 

CONCLUSIONS  AS  TO  GENESIS   OF  TONOPAH   ORES. 

The  considerations  above  pointed  out  appear  to  the  writer  to  indicate 
strongly  the  following  conclusions: 

The  Tonopah  district  was,  during  most  of  Tertiary  time,  a  region  of  active 
volcanism,  and  probably  after  each  eruption,  certainly  after  some  of  them, 
solfataric  action  and  fumarolic  action,  succeeded  by  hot  springs,  thoroughly 
altered  the  rocks  in  many  parts  of  the  district.  At  the  surface,  during  those 
periods,  the  phenomena  of  fumarolic  and  solfataric  action  and  of  hot  springs 
were  similar  to  those  to-day  witnessed  in  volcanic  regions;  but  the  rocks  now 
exposed  were  at  that  time  below  the  surface.  The  veins  fill  conduits  which  were 
formed  by  the  fractures  due  to  the  heavings  of  the  surging  volcanic  forces 
below  and  along  which  the  gases,  steam,  and  finally  hot  waters,  growing  gradually 

aSuess,  E.,  Geog.  Jour.,  vol.  20.  p.  520.  &  System  of  Mineralogy,  6th  ed.,  p.  20. 


262  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

cooler,  were  forced,  relieving  the  explosive  energies  of  the  subsiding  volcanism. 
The  water  and  other  vapors,  largely  given  off  by  the  congealing  lava  below, 
carried  with  them,  separated  and  concentrated  from  the  magma,  metals  of  such 
kind  and  of  such  quantities  as  are  present  in  the  veins,  together  with  silica  and 
other  materials. 

The  nature  of  the  metallic  minerals  in  the  veins  in  this  case  is  believed  to 
depend  largely  upon  the  particular  magma  whence  the  emanations  proceeded.  In 
the  chief  Tonopah  veins  this  was  the  earlier  andesite.  Other  factors,  such  as 
relative  depth,  have  evidently  an  important  controlling  influence. 


CHAPTER    VIII. 


INCREASE  OF  TEMPERATURE  WITH  DEPTH. 

Some  measurements  were  made  by  Mr.  Leon  Dominian,  field  assistant,  under 
the  direction  of  the  writer,  with  a  view  to  ascertaining  the  increment  of  temperature 
with  depth  in  this  district. 

METHOD  OF  MEASUREMENT. 

The  best  opportunities  were  offered  by  the  Mizpah  Extension  and  the  Ohio 
shafts,  both  fairly  deep  shafts  with  (at  that  time)  very  little  side  workings  and  no 
through  system  of  ventilation.  Holes  were  drilled  dry  into  the  rock  at  the  sides 
of  the  shafts  at  the  points  where  the  temperature  was  to  be  taken,  deep  enough 
to  take  in  the  thermometers,  which  were  especially  procured  for  this  purpose. 
After  the  thermometer  was  inserted  the  hole  was  stopped  up,  and  the  reading 
was  taken  after  fifteen  to  twenty-five  minutes  —  in  some  cases  twenty-four  hours. 
Check  measurements  were  taken  in  every  case.  In  the  Ohio  Tonopah  the  holes 
were  driven  18  inches;  in  the  Mizpah  Extension  not  so  deep. 

The  Ohio  Tonopah.  shaft  is  perfectly  dry.  The  Mizpah  Extension  encountered 
a  very  little  water  on  a  contact  zone  at  a  depth  of  300  feet,  but  is  otherwise 
quite  dry. 


TEMPERATURES 


THE  MIZPAHT  EXTENSION  AND  THE  OHIO 
TONOPAH. 


The  results  of  the  measurements  of  temperatures  are  given  in  the  following 

table: 

Temperatures  in  Mizpah  Extension  and  Ohio  Tonopah  shafts. 


Feet  below  surface. 

Temperature. 

Rate  of  increase  per  100 
feet. 

Depth  required  for  in- 
crease of  1  degree. 

Mizpah  Ex- 
tension. 

Ohio  Tono- 
pah. 

Mizpah  Ex- 
tension. 

Ohio  Tono- 
pah. 

Mizpah  Ex- 
tension. 

Ohio  Tono- 
pah. 

100                                 

Degrees  F. 
60.25 
61.75 
64 
66.5 
69 
70.5 
72 

Degrees  F. 
60 
61 
62.5 
64 
66.5 
69 
74 
78 

Degrees  F. 

Degrees  F. 

Feet. 

Feet. 

200              

1.5 
2.25 
2.5 
2.5 
1.5 
1.5 

1 
1.5 
1.5 
2.5 
2.5 
5 
7.6 

66§ 
44} 

40 
40 
66§ 
66§ 

100 
66§ 
66§ 
40 
40 
20 
16} 

300                             

400 

500                

600                              

700     

766  (bottom  Ohio  Tonopah) 

780  (bottom  Mizpah  Extension)... 

73.5 

1.9 

53J 

263 


264 


GEOLOGY    OK   TONOPAH    MINING    DISTRICT,   NEVADA. 


' 

Mizptih  Extension. 

Ohio  Tonopah. 

Average  increase  

1°  in  51.3  feet 

1°  iii  37  feet 

TEMPERATURES 


THE  MONTANA  TONOPAH. 


Some  observations  were  also  taken  in  the  Mizpah  and  in  the  Montana  Tonopah 
workings,  but  with  a  less  range  of  depth.  Those  in  the  Montana  Tonopah,  however, 
were  taken  at  intervals  along  the  vertical  shaft,  in  holes  drilled  for  the  purpose, 
and  the  thermometers  were  left  in  place  15  minutes,  check  readings  corresponding 
exactly.  They  are,  therefore,  worthy  of  confidence,  and  are  given  in  the  following 

table: 

Temperatures  in  Montana  Tonopah  shaft. 


Feet  below 
surface. 

Tempera- 
ture. 

Rate  of  in- 
crease per 
100  feet. 

Depth  re- 
quired for 
increase 
of  1°. 

Degrees  F. 

Degrees  F. 

Feet. 

317 

64 

460 

68 

2.8 

36 

600 

70.5 

1.8 

56 

Average  increase,  1°  in  43.5  feet. 

Although  the  average  increment  of  temperature  (1°  F.  in  43.5  feet)  for  the 
Montana  Tonopah  measurements  differs  from  that  shown  by  the  Mizpah  Extension 
measurements  (1°  in  51.3  feet),  comparison  of  the  tables  shows  that  the  temperatures 
for  the  corresponding  levels  in  each  case  practically  coincide. 

These  separate  temperature  measurements  have  been  plotted  as  curves  (fig.  77). 
The  Mizpah  Extension  curve,  as  shown,  is  distinct  from  the  Ohio  Tonopah  curve, 
while  the  Montana  Tonopah  curve  coincides  with  the  corresponding  portion  of 
the  Mizpah  Extension. 

TEMPERATURES  IN  MIZPAH   HILL.  WORKINGS. 

Six  measurements  were  taken  in  the  Mizpah  Hill  workings,  but  under  less 
exact  conditions.  They  were  taken  at  various  points  in  the  drifts,  and  so  at 
variable  distances  perpendicularly  from  the  surface,  sometimes  in  drilled  holes 
and  sometimes  at  the  ends  of  unventilated  drifts.  These  mines,  however,  had,  at 
the  time  of  examination,  a  thorough  system  of  ventilation  (which  the  others  did 
not)  and  the  measurements  do  not  check  exactly,  and  indicate  that  the  temperature 
was  affected  by  air  currents.  They  are  therefore  not  published. 


INCREASE    OF    TEMPERATURE    WITH    DEPTH. 


265 


THERMAL  SURVEYS  ON  THE  COMSTOCK. 

During  his  study  of  the  Comstock  Dr.  G.  F.  Becker  made  careful  thermal 
surveys  along  deep  vertical  shafts  and  along  the  Sutro  tunnel,  which  runs  in 
and  taps  the  vein.  On  plotting  the  temperatures  taken  in  the  shafts  no  indication 
of  curvature  could  be  perceived,  although  the  increment  showed  constant  local 
irregularities,  and  the  line,  plotted  from  point  to  point,  was  often  zigzag.  On 
this  account  a  straight  line  was  assumed  as  expressing  the  relation  of  temperature 
depth.  The  Sutro  tunnel  line,  however,  though  also  irregular  in  detail,  shows  an 
unmistakable  curve,  clearly  a  conduction  curve.  It  is  to  be  noted,  on  the  other 
hand,  that  in  the  Sutro  tunnel  the  temperature  measures  extended  over  a  distance 


20 


30 


Degrees  Fahrenheit 
40  50 


60 


80 


90 


Feet 

100 
200 

300 
400 
500 
600 
700 
SOO 
900 
1000 


Ohio  Tonopah 


Mizpah  Extension 


FIG.  77. — Plotting  of  temperature  observations  in  the  Ohio  Tonopah,  Mizpah  Extension,  and  Montana  Tonopah  mines, 
showing  increase  of  temperature  with  depth.    a=This  part  of  the  curve  coincides  with  that  of  the  Montana  Tonopah. 

of  11,000  feet,  while  the  vertical  shaft  measurement  did  not  extend  more  than 
2,000  feet;  and  that  any  given  2,000  feet  of  the  Sutro  tunnel  curve  would  not 
by  itself  suggest  a  curved  line. 

COMPARISON  OF  COMSTOCK  AND  TONOPAH  DATA. 

Comparing  the  Tonopah  and  Comstock  data,  the  temperature  of  78°  F., 
obtained  in  the  Ohio  Tonopah  at  766  feet  from  the  surface,  was  encountered  in  the 
Forman  shaft  at  the  Comstock  at  about  900  feet;  while  the  bottom  temperature  of 
73.5°  F.  in  the  Mizpah  Extension  at  780  feet  was  encountered  in  the  Forman  at 
between  600  and  700  feet.  It  seems  likely,  therefore,  that  the  average  increase  at 


266  GEOLOGY   OF   TONOPAH   MINING    DISTRICT,  NEVADA. 

Tonopah  ma}1  be  as  great  as  at  the  Comstock,  where  it  is  1°  F.  for  each  33  feet 
vertical  of  extent. 

The  decided  and  characteristic  curve  in  the  Ohio  Tonopah  has  no  counterpart 
in  any  of  the  vertical  sections  at  the  Comstock.  It  is  probably,  however,  a  local 
deviation  in  a  curve  of  vastly  greater  magnitude;  though  its  form  suggests  a  con- 
duction curve,  and  it  is  possible  that  the  extremely  rapid  increase  of  heat  at  the 
bottom  indicates  the  proximity  of  a  local  heat  focus,  such  as  a  hot  spring.  The 
larger  and  much  less  rapid  conduction  curve  of  the  Sutro  tunnel  section  is  due  to 
the  heat  from  a  similar  local  focus — the  hot  waters  which  rise  along  the  lode. 


CHAPTER    IX. 
COMPARISON  WITH  SIMILAR  ORE  DEPOSITS  ELSEWHERE. 

It  is  often  advisable  to  study  an  ore  deposit  or  a  mining  district  not  by  itself 
alone,  but  also  in  comparison  with  others.  Similar  districts  often  present  informa- 
tion, through  their  likeness  or  dissimilarities,  concerning  the  nature,  origin,  and 
future  possibilities  of  the  district  under  examination. 

VEIXS  OF  PACHUCA  AXD  REAL  DEL  MOXTE,  IX  MEXICO. 

Among  the  nearest  anajogies  to  Tonopah  yet  described  anywhere  in  the  world 
are  the  contiguous  mining  districts  of  Pachuca  and  Real  del  Monte,  described  by 
Aguilera  and  Ordonez. " 

These  celebrated  districts  are  62  miles  north  of  the  City  of  Mexico,  on  opposite 
slopes  of  the  Pachuca  Mountains,  which  bound  the  great  valley  of  Mexico.  The 
mines  support  the  city  of  Pachuca,  which  contains  35,000  people,  most  of  whom  are 
actually  engaged  in  mining.  The  ore  deposits  were  discovered  in  1522,  and  have 
been  worked  almost  continuously  to  the  present  day.  Pachuca  is  the  most  important 
mining  district  in  Mexico,  and  is  estimated  to  have  produced  since  its  discovery 
3,500,000  kilos  of  silver. b 

The  geology  is  similar  to  that  of  numerous  other  mineral  regions  of  Mexico. 
The  whole  Pachuca  Range  is  formed  of  Tertiary  andesites,  rhyolites,  and  basalts. 
The  andesites  are  of  Miocene  age  and  have  a  varied  appearance,  due  to  alteration, 
the  normal  type  being  green  and  propylitic.  The  feldspar  (labradorite)  has  often 
been  transformed  to  sericite,  calcite,  chlorite,  epidote,  and  clayev  products;  the 
pyroxene  to  chlorite,  viridite,  and  epidote.  The  rocks  are  silicined  near  the  veins, 
so  as  often  to  resemble  dacites  or  rhyolites,  this  alteration  being  due  to  the  influence 
of  hot  solutions  during  the  formation  of  the  veins.  Rhyolites  cover  the  andesites, 
occurring  as  flows  and  dikes.  The  last  eruptions  were  of  basalt.  The  veins  strike 
east  and  west.  Secondary  veins  branch  out  from  the  smaller  ones,  and  splitting  and 
reuniting  are  common  phenomena.  The  veins  are  more  remarkable  for  constancy 
and  extension  than  for  thickness.  They  seldom  exceed  20  feet  in  thickness,  while 
they  have  a  length  of  from  2£  to  10  miles. 

"Boletin  del  Institute  geo!6gico  de  Mexico,  Nos.  7,  8,  9:  Trans.  Am.  Inst.  Min.  Eng.,  vol.  32.  pp.  224-241. 
f>  About  112,000,000  ounces,  valued  at  8145,600,000  (1  oz.-about  81.30). 

267 


268  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

The  quartz  croppings  carry  pyrite  and  oxides  of  manganese.  They  are  always 
argentiferous,  with-  an  appreciable  amount  of  gold.  They  may  be  divided  into  two 
zones,  one  overlying  the  other.  The  upper  is  composed  of  oxides  (red  ores)  and  the 
lower  of  sulphides  (black  ores).  The  upper  contains,  besides  iron  oxide  (always 
auriferous),  oxides  of  manganese  and  chlorides  and  bromides  of  silver;  it  has  a 
maximum  downward  extent  of  nearly  1,000  feet.  The  lower  zone  contains  sul- 
phides of  lead,  silver,  etc.  The  lower  limit  of  the  upper  zone  corresponds  to  the 
ground-water  level. 

Calcite  is  found  only  in  small  quantities.  Of  the  sulphides,  pyrite,  galena, 
and  argentite  were  in  most  cases  deposited  simultaneously  with  the  quartz.  The 
abundant  manganese  oxide  in  the  upper  zone  is  replaced  in  the  sulphide  zone  by 
a  lesser  quantity  of  the  silicate,  rhodonite.  Pyrite  is  frequent  in  the  mineralized 
parts  of  the  veins,  and  is  also  abundant  in  the  country  rock,  but  in  the  country 
rock  it  does  not  contain  even  traces  of  the  precious  metals.  Native  silver  has  been 
found  at  all  depths;  ruby  silver  has  not  been  observed  at  Pachuca,  but  is  found 
at  Real  del  Monte.  a 

Rich  ores  occur  in  certain  parts  of  the  veins  called  bonanzas,  which  are  of 
irregular  form,  frequently  nearly  elliptical.  The  bonanzas  of  the  different  veins 
group  themselves  in  a  northeast-southwest  zone  nearly  normal  to  the  vein  strike. 
Some  are  in  the  oxidized,  some  in  the  sulphide  zone;  the  former  are  more 
numerous.  In  some  cases  bonanzas  were  encountered  at  the  surface;  in  others 
they  were  found  in  depth,  where  the  vein  was  barren  at  its  outcrop.  The  size 
of  the  bonanzas  varies;  one  of  the  largest  was  encountered  at  a  depth  of  over  300 
feet  and  was  elliptical,  the  greatest  axis  being  over  3,000  and  the  smaller  1,300 
feet,  with  a  thickness  of  8  feet. 

The  veins  become  impoverished  at  great  depths.  At  the  bottom  they  change 
to  barren  galena  and  blende,  too  poor  to  repay  working.  However,  certain 
developments  lead  to  the  belief  that  at  still  greater  depth  new  bonanzas  might 
be  found.  Most  of  the  mines  are  only  about  1,300  feet  or  less  deep;  in  only 
one  has  a  little  work  been  done  as  deep  as  1,800  feet. 

This  district  is  similar  to  Tonopah  in  the  character  and  age  of  the  wall  rocks 
(Miocene  andesites);  in  the  nature  of  the  alteration  of  the  rock  near  the  veins 
(silicification  near  the  veins,  propylitic  alteration  farther  away);  in  the  structural 
characters  of  the  veins,  which  form  a  splitting  and  reuniting  group;  in  the 
general  character  of  ores  (both  oxide  and  sulphide),  and  of  gangue,  though 
adularia  as  a  gangue  material  and  selenides  as  ores  have  not  been  recognized  at 
Pachuca;  and  in  the  occurrence  of  the  rich  ores  in  bonanzas,  which  seems  to  be 
due  to  the  intersection  of  transverse  fractures  with  the  main  vein  zone. 


"  Be<;k,  Richard,  Erzlagerstiittcn,  2d  ed.,  p.  '277. 


COMPARISON    WITH    SIMILAR    ORE    DEPOSITS    ELSEWHERE.  269 

OTHER  SIMILAR  MIXERAL,  DISTRICTS  IX  MEXICO. 

The  deposits  of  Pachuca  are  similar  in  many  respects  to  many  other  Mexican 
ores.  J.  G.  Aguilera  remarks  concerning  the  ores  of  Mexico  in  general: 

"The  silver  deposits  proper  are  found  in  eruptive  rocks.  A  very  few  are  found 
in  sedimentary  rocks,  and  in  these  the  silver  is  accidental  and  variable  in  quantity. 
Where  silver  veins  occur  in  sedimentary  rocks  it  is  evident  that  they  are  related  to 
and  dependent  upon  andesitic  Tertiary  eruptive  rocks."" 

"The  majorit\r  of  the  silver- veins  of  Mexico  are  in  hornblende-  and 
pyroxene-andesite.  As  examples  of  fissure  veins  in  eruptive  andesitic  rocks,  we  mav 
mention  the  following:  In  Zopilote,  Tepic,  the  veins  have  a  northwest  course,  and 
consist  of  quartz,  blende,  and  pyrite,  sulphides  of  silver,  and  small  amounts  of 
galena.  At  Topia  the  veins  extend  northeast-southwest,  and  contain  galena,  blende, 
a  very  small  amount  of  pyrite,  argentite,  and  pyrargyrite  with  a  gangue  of  quartz 
and  calcite.  At  the  mines  of  Tecatitliin,  Jalisco,  the  veins  strike  about  N.  40°  W., 
and  dip  45°  to  the  southwest.  The  gangue  is  quartz  with  a  little  calcite,  carry- 
ing sulphides  and  antimonides  of  silver,  pyrite,  and  chalcopyrite.  At  Chinipas, 
Chihuahua,  the  veins  occur  in  diorite  and  hornblende-andesite.  The  strike  is 
northeast,  or  in  some  cases  northwest.  The  vein  filling  is  quartz  with  argentite  and 
pyrite,  oxides  of  iron,  and  dendritic  manganese.  At  Ajijic,  Jalisco,  the  veins  are  in 
hornblende-andesite,  with  an  east-west  strike;  there  is  an  oxidized  zone,  and  as  depth 
is  reached  complex  sulphides  are  encountered.  At  San  Sebastian  and  Los  Reyes, 
Jalisco,  the  veins  have  a  quartz  gangue  with  some  calcite,  complex  sulphides,  and 
tellurides  of  silver  and  gold,  a  very  little  galena,  blende,  and  pyrite.  The  veins  of 
the  Rosario  mines  and  San  Nicolas  del  Oro  mine,  Guerrero,  are  in  hornblende- 
andesite;  their  course  is  northwest,  or  in  some  cases  northeast,  and  they  contain  an 
oxidized  zone.  Below  this  is  the  sulphide  zone,  containing  argentite,  ruby  silver, 
pvrite,  and  a  small  amount  of  chalcopyrite.  The  gangue  is  quartz,  carrying  gold. 
Some  of  the  veins  of  Sierra  de  Tapalpa,  San  Jose  del  Amparo,  and  Rosario,  etc., 
have  a  north-south  course,  and  dip  west;  the  gangue  is  quartz  with  some  barite.  In 
the  oxidized  zone  they  contain  the  carbonates  of  copper,  and  beneath  this  grav 
copper  and  stibnite  occur.  At  Tlalchapa,  Guerrero,  the  lodes  have  a  northwest- 
southeast  course,  dipping  to  the  northeast.  The  vein-filling  is  quartz  with  argentite, 
pvrite,  and  blende;  occasionally  the  vein  quartz  contains  calcite  and,  in  addition  to 
the  minerals  named  above,  galena  and  chalcop\Trite.  At  the  mines  of  Chacoaco, 
south  of  Fresnillo,  the  veins  extend  nearly  north  and  south,  and  contain  quartz  with 
marcasite  and  pyrite.  Some  of  the  veins  strike  northeast-southwest,  and  contain 
quartz,  pyrite,  and  sulphides  of  silver.  The  veins  of  Real  del  Espiritu  Santo  are 
found  in  augite-andesite. 

"In  the  pyroxene-andesites  may  be  found  the  deposits  of  Pachuca,  Real  del 
Monte,  El  Chico,  Tepenene,  Capula,  Santa  Rosa,  in  Hidalgo;  the  mines  of  Santo 
Domingo,  in  Jalisco;  and  some  of  the  mines  of  Noxtepec,  Guerrero.  Among  the 
veins  in  andesite  may  be  mentioned  those  of  the  following  mines:  San  Pable  Analco, 
which  in  the  oxidized  zone  somewhat  resembles  those  of  Pachuca;  the  California 


"Trans.  Am.  Inst.  Min.  Eng.,  vol.  32,  p.  513. 


270  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 

mines,  in  which  part  of  the  veins  strike  northeast  and  dip  southeast  and  others  have 
their  course  toward  the  northwest  and  dip  northeast.  The  gangue  is  quartz,  carrying 
galena,  pyrite,  chalcopyrite,  and  tetrahedrite.  In  the  San  Rafael  mine,  Jalisco,  the 
veins  have  a  course  N.  25°  W.  In  the  mines  of  Hostotipaquillo  the  veins  contain 
calcite  and  quartz  with  some  rhodochrosite,  a  small  amount  of  pyrite  and  black  blende, 
argentite,  galena,  chalcocite,  and  chalcopyrite.  In  the  oxidized  zone  they  contain 
native  silver,  carbonates  of  copper,  and  a  very  small  amount  of  copper  oxide.  It 
would  be  tiresome  to  enumerate  all  the  silver  veins  of  Mexico  which  occur  in 
andesites,  but  as  has  been  said,  the  majority  of  the  silver  veins  of  the  country  are  in 
various  species  of  this  rock,  which  Humboldt  designated  as  metalliferous  porphyries."0 
Rarely  similar  veins  are  found  in  rhyolite.6 

Perusal  of  the  instances  mentioned  above  by  Aguilera  shows  that  the  veins  are 
all  closely  alike,  not  only  in  regard  to  their  country  rock,  but  to  their  tilling. 

THE  COMSTOCK  LODE. 

Pachuca  is  about  2,000  miles  southwest  of  Tonopah,  but  a  similar  analogous 
deposit  (the  Comstock)  lies  150  miles  to  the  northwest. 

The  Comstock  lode  is  a  vein  4  miles  long  which  has  formed  in  Tertiary  eruptive 
rocks,  chiefly  andesites,  along  a  fault  line  having  a  maximum  displacement  of  3,000 
feet.  At  both  ends  it  branches  and  so  dies  out.  It  strikes  east  of  south  and  dips 
east.  It  was  discovered  in  1859,  and  worked  up  till  the  present  day,  but  most 
actively  from  1861  to  1880.  Up  to  June,  1902,  it  had  yielded  $369,566,112.61  worth 
of  ore,  of  which  about  42^  per  cent  was  gold  and  57£  per  cent  silver/  The  rocks  of 
the  district  in  the  order  of  their  succession  are,  according  to  Hague  and  Iddings/ 
andesite,  dacite,  rhyolite,  andesite,  and  basalt.  The  andesites  are  coarse  grained  in 
depth  (diorites  and  diabases).  Near  the  lode,  and  for  some  distance  away,  in  a  space 
about  5  by  2  miles,  the  country  rock  (chiefly  andesitic)  is  extremely  decomposed, 
the  period  of  alteration  having  succeeded  an  andesitic  eruption.  The  hornblende, 
augite,  and  biotite  have  altered  to  chlorite,  pyrite,  epidote,  etc.,  the  feldspar  to  quartz 
and  an  undetermined  white  aggregate.  This  altered  andesite  is  the  famous  "propy- 
lite."  The  basalt,  which  is  the  latest  rock  of  the  district,  has  not  been  altered  in  the 
same  way  as  the  andesites.  The  alteration  of  the  rocks  and  the  lode  was  due  to 
solfataric  action  which  accompanied  the  faulting. 

The  lode  material  is  quartz,  certain  limited  portions  of  which  contained  large 
quantities  of  silver  and  gold  (bonanzas),  while  the  rest  is  low  grade.  Calcite  is 
much  less  than  quartz  in  amount  and  is  generally  insignificant.  Most  of  the  bullion 
has  been  derived  from  a  bluish  quartz,  like  that  at  Tonopah,  the  color  being  mainly 

"Trans.  Am.  In»t.  Min.  Eng.,  vol.  32,  pp.  515-516. 
6  Ibid.,  p.  517. 

e  Becker,  G.  F..  Mon.  f.  S.  Oeol.  Survey,  vol.  3,  pp.  9, 11.    Also  Kept,  of  the  Director  of  the  Mint  for  1901,  p.  169. 
>l  Hague,  A.,  and  Iddlngs,  J.  R.,  Bull.  U.  S.  Oeol.  Survey  No.  17.    Doctor  Becker's  determinations  anil  succession  are 
somewhat  different,  a»  follows:  Granite,  diorite,  quartz-porphyry,  diabase,  andesitc,  basalt. 


COMPARISON    WITH    SIMILAR    ORE    DEPOSITS    ELSEWHERE.  271 

due  to  disseminated  argentite,  which  is  the  principal  ore  mineral  and  is  accompanied 
by  gold,  probably  free.  Bunches  of  stephanite,  polybasite,  and  ruby  silver  were  also 
found.  In  the  bonanzas,  near  the  surface,  chlorides  and  native  silver  occurred. 
Frequently  the  ore  grew  base,  and  carried  large  quantities  of  galena,  zinc  blende,  etc. 

Pyrite  occurs  abundantly  both  in  the  altered  country  rock  and  in  the  ore.  The 
mineralizing  solutions  are  thought  to  have  derived  their  heat  from  volcanic  rocks, 
and  thus  the  general  phenomena  are  classed  as  due  to  solfataric  action,  but  the 
materials  precipitated,  including  the  ores,  are  thought  to  have  been  derived  from 
the  decomposed  wall  rock. 

The  workable  bodies  or  bonanzas  represent  the  smaller  portion  of  the  lode. 
The  value  of  the  ore  in  them  ranges  from  $15  a  ton  to  (very  locally)  several 
thousand  dollars.  They  are  encountered  at  various  depths,  from  the  surface  down 
to  3,000  feet.  The  vein  down  to  nearly  2,000  feet  contained  16  workable  ore 
bodies,  while  below  this  level  the  ore  has  proved  mostly  low  grade.  One  large 
bonanza  (that  of  the  C.  &  C.  and  Con.  Virginia)  extends  vertically  from  about 
1,250  to  1,950  feet  below  the  surface,  and  has  a  greatest  diameter  of  about  1,100 
feet.  It  yielded  about  one-tenth  the  product  of  the  lode."  The  ore  minerals 
were  chiefly  stephanite,  argentite,  and  gold,  the  latter  probably  free. 

The  source  of  the  heated  waters  which  are  encountered  in  the  mines,  and 
which  are  thought  to  have  accomplished  the  rock  alteration  and  ore  deposition,  is 
concluded  from  thermal  surveys  to  be  not  less  than  2  miles  deep,  and  the  heat 
and  the  active  reagents,  such  as  carbonic  and  sulphydric  acids,  are  thought  to 
have  had  a  volcanic  origin,  while  the  waters  may  have  had  an  atmospheric  source. 
The  waters  above  800  feet  had  a  temperature  of  about  70°  F.,  while  from  about 
1,000  feet  down  hot  waters  of  above  100°  F.,  rising  under  pressure,  were 
repeatedly  encountered. 

The  Comstock  district  is  similar  to  Tonopah  in  respect  to  the  character  and 
age  of  the  rocks  in  which  the  lode  lies  (Tertiar}-  andesites),  in  their  "propy- 
litic"  alteration,  in  the  nature  of  the  gangue  and  ore,  and  in  the  occurrence  of 
the  rich  ores  in  irregular  "  bonanzas."  The  chief  distinction  is  that  the  Comstock 
consists  of  a  single  very  strong  lode,  while  at  Tonopah  there  are  a  number,  of 
less  size. 

SILVER  CITY  AXD  DE  1AMAR  DISTRICTS,  IDAHO. 

Another  region  having  many  striking  peculiarities  in  common  with  Tonopah 
lies  about  400  miles  due  north  of  Tonopah.  The  districts  of  Silver  City  and  De 
Lamar  (5  miles  apart)  are  situated  in  the  Ohwyee  Range,  in  southwestern  Idaho.* 
The  range  has  a  granite  core,  almost  covered  by  Miocene  rhyolite  and  basaltic 

n  This  ore  averaged  about  $80  per  ton,  with  silver  at  31.29  per  ounce. 
fcLindgren,  W.,  Twentieth  Ann.  Kept.  U.  S.  Geol.  Survey,  pt.  3,  pp.  107-188. 


272  GEOLOGY    OF   TON  OP  AH    MINING    DISTRICT,   NEVADA. 

lavas.  The  ores  were  discovered  in  1863.  The  total  production  to  1899  was 
313,448  ounces  gold  and  10,540,000  ounces  silver.  The  deposits  are  normal 
fissure  veins,  chiefly  in  rhyolite.  In  one  type  the  principal  ore  minerals  are 
small  quantities  of  argentite  and  chalcopyrite,  with  a  gangue  of  quartz  and  ortho- 
clase  (adularia).  The  proportion  of  gold  to  silver  by  weight  averages  1:120. 
In  the  other  type  scarcely  any  sulphides  are  ordinarily  visible,  though  occasion- 
ally pyrite,  argentite,  and  pyrargyrite  occur.  The  gangue  is  quartz,  pseudo- 
morphic  after  calcite  or  barite.  The  relation  of  gold  to  silver  by  weight  is  about 
1 : 10.  At  De  Lamar  there  is  a  strong  silicification  of  the  country  rock  near 
the  veins,  with  the  formation  of  abundant  pyrite  and  marcasite,  and  a  little 
sericite.  Farther  away  from  the  veins  the  country  rock  is  softer  and  more 
pyritized.  The  veins  strike  northwest  and  dip  southwest,  both  strike  and  dip 
varying  considerably.  The  system  comprises  ten  veins,  20  to  80  feet  apart.  The 
strike  of  these  veins  is  such  that  parts  of  the  group  are  like  some  of  the  radii 
of  a  circle,  as  is  the  case  at  Tonopah,  and  each  vein  may  join  and  fork  in  the 
manner  of  linked  veins,  both  horizontally  and  vertically.  The  width  of  the  veins 
is  from  1  to  6  feet,  averaging  3  or  4  feet.  The  rich  ore  occurs  in  large,  contin- 
uous bodies  extending  from  the  surface  to  a  depth  of  a  1,000  feet,  dipping 
gently  (20°-30°)  southeastward  along  the  plane  of  the  vein.  They  are  generally 
about  200  feet  long  arid  ordinarily  1  to  6  feet  thick. 

In  other  veins  the  ore  bodies  do  not  extend  so  deep,  and,  while  having  often 
a  generally  definite  course,  are  so  irregular  and  discontinuous  as  to  constitute 
irregular  bonanzas  rather  than  definite  shoots."  No  considerable  ore  shoots  have 
been  yet  found  below  1,000  feet,  though  the  veins  remain  strong.  Cerargyrite, 
pyrargyrite,  and  argentite  occur  locally  (the  latter  being  common  to  nearly  all 
the  mines),  as  well  as  polybasite,  proustite,  native  gold  and  silver. 

Besides  occurring  in  rhyolite,  some  of  the  veins  are  also  in  granite  and  basalt. 

The  rock  alteration  and  the  ore  deposition  are  considered  to  have  been  accom- 
plished by  ascending  hot  waters,  whose  nature  is  indicated  by  the  silicification  of 
the  rhyolite  and  the  formation  of  adularia,  chlorite,  and  epidote.  The  period  of 
formation  is  post-Miocene.  The  veins  extend  along  the  strike  sometimes  for  a 
mile  or  so,  but  average  less;  they  die  out  on  botli  ends.  The  ore  at  present  mined 
at  De  Lamar  goes  $14  in  gold  and  $2  in  silver;  in  1872  the  average  value  of  the 
ore  mined  was  from  $12  to  $t>0  per  ton  in  different  mines. 

The  districts  of  Silver  City  and  De  Lamar  just  described  are  similar  to  Tonopah 
in  that  the  ores  occur  in  Tertiary  volcanics,  and  are  probably  in  both  cases  post- 
Miocene  in  age;  to  a  striking  degree  in  the  character  of  the  ores  and  gangue 
materials;  in  the  structural  character  of  the  veins,  which  form  a  group  knit  together 

a  Op.  cit.,  p.  152. 


COMPARISON    WITH  ^SIMILAR    ORE    DEPOSITS    ELSEWHERE.  273 

by  branches;  in  the  general  character  of  the  alteration  of  the  wall  rock;  and  in  the 
occurrence  of  the  rich  ores  in  irregular  bonanzas.  The  chief  difference  is  that  the 
wall  rocks  are  mainly  rhyolite  and  not  andesite. 

RELATION  OF  THE  DESCRIBED  DISTRICTS  TO  TONOPAH. 

Of  all  the  described  ore  deposits  of  North  America,  therefore,  Tonopah  appears 
to  be  most  closely  related  to  many  of  the  Mexican  silver  veins,  and  also  to  the 
Comstock  in  Nevada  and  the  Silver  City-De  Lamar  veins  of  Idaho.  With  Pachuca, 
as  is  seen,  the  relation  is  intimate,  and  Ordonez's  description  of  the  veins  of  this 
district  would  do,  with  a  very  little  change,  for  a  report  on  the  Tonopah  veins. 
The  chief  difference  is  in  the  occurrence  of  manganese  silicate  in  depth  at  Pachuca. 
which  has  not  been  found  at  Tonopah, "  and  also  the  less  content  of  gold,  with 
the  absence  of  ruby  silver.  Ruby  silver,  however,  occurs  in  the  cognate  and 
contiguous  Real  del  Monte  district;  also  gold  in  considerable  quantity  occurs 
with  silver  in  some  of  the  Mexican  districts  of  this  type.  Those  enumerated  bv 
Aguilera*  all  occur  in  hornblendic  andesite. 

This  group  of  veins  is  characterized  by  the  following  features:  They  occur 
in  Tertiary  volcanic  rocks  of  similar  character  in  the  different  localities,  being 
chiefly  Miocene  andesites  or  rhyolites.  They  constitute  strong  masses  or  frequently 
branching  and  "linked"  veins  of  quartz,  which  have  as  gangue  essentially  quartz, 
with  frequently  a  little  calcite,  while  adularia,  barite,  rhodochrosite,  or  rhodonite 
may  also  be  present  in  limited  amount.  The  ore  is  characteristically  a  silver- 
gold  one,  silver  being  usually  predominant  in  the  values  in  vaiying  proportions, 
though  the  relative  value  may  be  -reversed,  and  in  some  extreme  cases  either 
metal  may  occur  with  little  admixture  of  the  other.  In  any  case  the  abundance 
of  silver  or  gold,  or  both,  in  reference  to  lead,  zinc,  iron,  etc.,  is  characteristic. 
Silver  sulphides,  especially  argentite,  also  stephanite  and  polybasite  (with  ruby 
silver)  and  gold,  probably  largely  in  the  free  state,  are  distinguishing  features 
in  the  great  majority  of  cases.  Telluridesc  and  selenides  may  also  be  present. 
Pyrite,  blende,  chalcopyrite,  and  galena  are  usually  present  in  varying  quantity. 
Where  they  become  predominant  the  vein  becomes  relatively  low  grade. 
Tetrahedrite,  stibnite,  and  bismuthinite''  are  also  known  to  occur.  The  wall 
rocks  are  characteristically  much  altered  to  quartz,  sericite,  chlorite,  calcite, 
epidote,  pyrite,  and  sometimes  adularia,  etc.  Frequently  the  rocks  nearest  the 
veins  are  chiefly  altered  to  quartz  and  sericite,  those  farther  away  to  the  softer 
"propylitic"  alteration,  consisting  of  calcite,  chlorite,  pyrite,  epidote,  etc. 

a  Since  the  above  was  written  manganese  carbonate  has  been  found  in  the  sulphide  ores  at  Tonopah.    See  p.  89. 

6  Aguilera,  J.  G.,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  32,  p.  519. 

oAt  Goldfield,  Nev.,  and  Jalisco  and  Tepic  in  Mexico  (Trans.  Am.  Inst.  Min.  Eng.,  vol.  32,  p.  601). 

<iAt  Goldfleld.    See  Bull.  U.  S.  Geol.  Survey  No.  260,  p.  138. 

16843— No.  42—05 18 


274  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 

The  rich  ores  occur  in  irregularly  outlined  portions  of  the  lode  called 
bonanzas.  These  bonanzas  are  of  limited  extent  both  horizontally  and  vertically. 
They  are  believed  to  have  arisen  as  a  consequence  of  the  irregular  intersection 
of  transverse  fractures  or  tissure.s  with  the  main  vein  channel,  producing  maximum 
deposition  in  these  portions.  Intervening  portions  may  be  low  grade  or  barren. 

In  the  oxidized  zone,  silver  chlorides  and  bromides,  free  gold,  manganese 
oxide,  etc.,  occur. 

THE  PETKOGKAPHIC  PROVINCE  OF  THE  GREAT  BASIN. 

After  a  study  of  the  lavas  of  the  Great  Basin  region  of  Nevada  in  1900  the 
writer"  came  to  the  conclusion  that  the  whole  region  "southward  into  the 
Mojave  Desert,  together  with  a  portion  at  least  of  the  Sierra  Nevada,  constitutes 
a  petrographic  province;  that  is  to  say,  it  is  underlain  by  a  single  body  of  molten 
magma,  which  has  supplied,  at  different  periods,  lavas  of  similar  composition  to 
all  the  different  parts  of  the  overlying  surface.  The  limits  of  this  subcrustal 
basin,  however,  are  not  yet  defined  in  any  direction." 

The  general  sequence  of  lavas,  roughly  outlined,  was  concluded  to  be  as  follows: 

1.  Rhyolite  (Eocene). 

2.  Andesite  (Miocene). 

3.  Rhyolite  with  occasional  basalt  (Miocene-Pliocene). 

4.  Andesite  (Late  Pliocene-Early  Pleistocene). 

5.  Basalts  and  occasional  rhyolites  (Pleistocene). 

EXTENSION  OF  THE  GREAT  BASIN  PETROGRAPHIC    PROVINCE   INTO 

MEXICO. 

Later  in  the  same  3'ear,  Ordonez,  in  a  stud}-  of  the  rhyolites  of  Mexico6  over 
a  northwesterly  trending  belt  extending  from  the  northern  boundary  southward 
past  the  City  of  Mexico,  found  that  the  author's  conclusions  were  also  applicable 
to  this  province.  He  writes  as  follows: 

With  very  slight  differences,  which  are  without  decisive  importance,  one  may 
say  that  everywhere  the  relative  order  of  eruptions,  judging  from  the  composition 
and  structure  of  the  rocks,  has  been  the  same.  Let  us  here  present  the  example  of 
the  Great  Basin  of  Nevada.  Many  ranges  of  that  region  show  a  succession  strictly 
comparable  with  that  of  Mexico. 

The  general  succession  is  found  to  correspond  with  that  given  by  the  writer 
above,  and  the  rhyolites  occupy  the  same  position  and  are  of  the  same  age  (Miocene- 
Pliocene)  as  those  under  No.  3.  The  andesites,  which  preceded  the  rhyolites, 
correspond  with  No.  2,  and  are  Miocene. <•' 

ogpurr,  J.  E.,  Jour.  Geol.,  vol.  8,  1900,  p.  (S38. 

fcOrdoflez,  E.,  Boletln  del  Instiluco  geo!6gieo  de  Mexico,  No.  14,  p.  66. 

"Op.  oil.,  p.  67. 


COMPARISON    WITH    SIMILAR    ORE    DEPOSITS    ELSEWHERE.  275 

PROBABLE  STILL  FURTHER  EXTENSION  OF  THE   GREAT  BASIN- 
MEXICO  PETROGRAPHIC  PROVINCE. 

In  1902  the  author"  recalled  his  description  of  the  petrographic  province, 
which  includes  the  volcanic  region  of  Nevada,  and  noted  the  work  of  Ordonez. 
He  also  called  attention  to  the  fact  that  later  developments  showed  similar  lavas 
of  similar  age  and  succession  in  localities  in  the  State  of  Washington  and  on  the 
California  coast.  His  statement  was  as  follows: 

"  Without  being  in  danger  of  carrying  this  correlation  to  excess  I  may  point 
out  that  the  Pliocene  olivine-basalts  of  the  Sierra  Nevada*  are  abundantly  present 
in  Oregon  and  Washington;  that  the  British  Columbia  basalts  are  approximately, 
at  least,  of  the  same  period/  and  that  throughout  the  whole  of  Alaska  and  into 
the  Bering  Sea  occur  olivine-basalts  of  Pliocene  age.'' 

"  Again,  the  abundance  of  basic  andesities  (typically  augitic,  often  hypersthene- 
bearing,  and  verging  toward  basalts)  all  belonging  to  one  epoch  (very  late 
Pliocene-Pleistocene),  in  a  continuous  belt  in  Alaska,  running  the  whole  length  of 
the  Aleutian  Islands  and  peninsula,  turning  the  same  angle  as  the  chief  orographic 
and  topographic  features,  and  running  down  the  coast  past  Sitka;e  the  occurrence 
of  the  same  rocks,  belonging  to  the  same  age,  in  Washington  and  Oregon 
(Mount  Rainier,  etc.);  the  extension  of  the  belt  through  the  Sierra  Nevada 
and  along  the  western  part  of  the  Great  Basin;  finally  its  extension  into  Mexico-''— 
this  is  all  striking  and  deserves  recognition.  Moreover,  this  belt  of  late  Pliocene- 
Pleistocene  augite  (hypersthene)  andesites  extends  through  Central  and  South 
America,  in  the  Andes.9'  In  Alaska  and  in  the  Andes  some  of  the  cones  of  this 
epoch  are  still  active,  but  the  majority  have  become  extinct. 

"It  appears,  then,  that  the  whole  extreme  western  part  of  the  western 
hemisphere  (the  Pacific  coast  of  the  Americas)  is  a  zone  occupied  by  what  (at  some 
periods,  at  least)  is  and  has  been  a  single  petrographic  province. 

"It  remains  to  be  seen  whether  this  province  is  not  continued  into  Asia  with 
the  change  of  erogenic  trends  in  Alaska  from  northwest  to  southwest.  The  line  of 
late  Tertiary-Pleistocene  volcanoes,  which  extends  along  the  Aleutian  Islands  to 
Kamchatka,  is  represented  by  15  or  20  cones  in  this  peninsula;  this  line,  following 
the  general  erogenic  trend,  runs  southwest  through  the  Kurile  Islands,  the  islands 
of  Japan,  and  the  Philippines,  into  the  East  Indies.  Andesites — largely  pyroxene 
andesites,  and  frequently  hypersthene  andesites — are  characteristic  of  this  chain  also, 
as  far  as  the  famous  volcano  of  Krakatua." 


ngpurr,  J.  E.,  Trans.  Am.  Inst.  Min.  Eng.,  vol.  33,  pp.  332-333. 
fcSpurr,  J.  E.,  Jour.  Geol.,  vol.  8,  No.  7,  chart,  p.  643. 

cDawson,  G.  M.,  Ann.  Kept.  Geol.  Nat.  Hist.  Survey  Canada,  vol.  3,  pt.  1,  p.  37,  B;  also,  Trans.  Royal  Soc.  Canada,  vol.  8, 
sec.  4,  p.  15.         i 

<i  Spurt,  J.  E.,  Geology  of  the  Yukon  gold  district.  Eighteenth  Ann.  Kept.  U.  S.  Geol.  Survey,  pt.  3,  p.  250. 
<•  Spurr,  J.  E.,  Reconnaissance  in  southwestern  Alaska,  Twentieth  Ann.  Rept.  U.  S.  Geol.  Survey,  pt.  7,  map  13. 
1  Ordonez,  Ezequiel,  Las  rhyolitas  de  Mexico,  Boletin  del  Institute  geo!6gico  de  Mexico,  No.  14,  p.  66. 
uZirkel.  Lehrbuch  d.  Petrographie,  2d  ed.,  vol.  2,  pp.  831-832. 


276  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,    NEVADA. 

A    METALLOGRAPHIC    PROVINCE    COEXTENSIVE    WITH    THE    PETRO- 

GRAPHIC    PROVINCE. 

In  the  paper  above  referred  to  the  writer  brought  forward  the  idea  of  metal- 
liferous provinces  (perhaps  better,  metallographic  provinces)  characterized  by  the 
presence  of  certain  metals;  and  pointed  out  that  these  provinces  may  or  may  not 
be  closely  identified  with  petrographic  provinces,  although  they  probabhT  generally 
are  so,  to  a  certain  extent  at  least." 

Unquestionably  the  close  relation  between  the  Nevada  mineral  districts,  Tonopah 
and  the  Cornstock,  with  the  far  more  numerous  array  in  Mexico,  and  the  individuality 
of  this  group  as  compared  with  other  known  veins  of  North  America,  shows  a 
metallographic  province,  which  in  this  case  coincides  with  a  portion  of  the  petro- 
graphic province  previously  mentioned. 

In  this  metallographic  province  ores  occur  in  Miocene  andesites  in  the  great 
majority  of  cases,  and  their  formation  followed  soon  after  the  eruption  of  these 
rocks.  In  occasionally  recurring  cases  (such  as  Silver  City  and  De  Lamar,  Idaho, 
and  others)  they  appear  in  Miocene-Pliocene  rhyolites,  which  succeeded  the  andesites. 

In  general,  however,  the  Miocene  andesites  of  this  province  are,  as  Humboldt 
noted,  the  metalliferous  formation  par  excellence,  and  if  the  conclusions  which  have 
been  arrived  at  regarding  Tonopah  are  correct  (which  coincide  with  a  number  of 
similar  conclusions  concerning  other  districts  reached  by  other  authors),  the  ore  is 
due  to  the  after  actions  of  the  eruptions  in  the  shape  of  fumaroles,  solfataras,  and 
hot  springs.  Moreover,  since  similar  manifestations  (of  fumaroles,  solfataras,  and 
hot  springs)  follow  most  volcanic  eruptions,  it  is  probable'that  the  metals  deposited 
by  the  after  processes  at  this  period  arose  from  an  unusual  proportion  of  them  in 
the  andesitic  magma;  indeed,  the  very  definition  of  a  metallographic  province 
implies  this.  The  existence  of  such  metallographic  provinces  is  evident;  and  the 
theory  of  their  origin,  as  propounded  by  the  writer,  is  like  that  long  entertained 
by  many  petrographers  for  the  origin  of  petrographic  provinces — namely,  that 
the}'  are  formed  by  magmatic  segregation.  * 

ORIGIN    OF    SHOOTS    OR    BONANZAS    IN    THE    VEINS    OF    THIS 
METALLOGRAPHIC    PROVINCE. 

Light  is  thrown  upon  the  origin  of  the  shoots,  chimneys,  or  bonanzas  in 
this  class  of  veins  by  the  studies  of  the  influence  of  cross  fractures  on  their 
formation  in  Tonopah,  and  the  similarity  between  these  bonanzas  and  those  at 
Silver  City  and  De  Lamar,  Idaho,  the  Cornstock  and  Pachuca  (fig.  78).  At  De 
Lamar  the  shoot  or  chimney  form  is  evident,  some  of  the  bonanzas  having  been 


"Trans.  Am.  last.  Min.  Kng.,  vol.  33,  p.  33f>. 

fcSpurr,  J.  E.,  Trans.  Am.  lust.  Mln.  Eng.,  vol.  S3,  p.  336. 


COMPARISON    WITH    SIMILAR    ORE    DEPOSITS    ELSEWHERE. 


277 


followed  downward  over  a  thousand  feet,  yet  the  local  irregularity  of  the  outline 
is  like  that  of  the  typical  bonanza.  At  Tonopah  a  similar  shoot-like  form  with 
a  definite  pitch  has  been  discerned,  but  the  developments  thus  far  made  do  not 


(B) 


FIG.  78.— Vertical  cross  sections  showing  forms  of  ore  bodies  or  bonanzas  in  districts  similar  to  Tonopah.  (A )  Vertical  section 
of  Poor  Man  and  Silver  Cord  veins,  showing  extent  of  rich  ore  body  in  De  Lamar  district;  after  Lindgren,  Twentieth 
Ann.  Kept.  U.  S.  Geol.  Survey,  pt.  3,  p.  152.  (B)  Portion  of  projected  vertical  section  of  the  Comstock  lode,  Nevada, 
showing  some  of  the  chief  bonanzas  on  the  vein;  adapted  from  Becker,  Men.  U.  S.  Geol.  Survey,  vol.  3,  atlas. 
(O  Projected  vertical  section  of  a  portion  of  the  Cristo  vein,  Pachuca,  Mexico,  showing  bonanzas  on  the  vein:  after 
Aguilera  and  Ordonez,  Boletin  del  Instituto  geo!6gico  de  Mexico,  Nos.  7,  8,  and  9. 

show  so  great  a  persistency  as  at  De  Lamar.  At  Tonopah  the  connection  of  the 
shoots  with  cross  fractures  is  evident,  and  the  localization  of  the  ore  deposition 
at  intersections  of  especially  fractured  zones  seems  the  correct  explanation.  It 


278  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

is  doubtful,  however,  if,  when  the  bonanzas  in  the  Tonopah  veins  shall  have  been 
worked  out,  the  shoot-like  form  will  always  be  discernible;  in  the  case  of  the 
richer  eastward-pitching  shoots  of  the  Mizpah  vein,  for  example,  the  spaces 
between  the  shoots  should  probabty  be  considered  together  with  them,  in  the 
larger  sense,  as  parts  of  one  great  bonanza,  whose  eastward  pitch  and  shoot-like 
form  would  be  less  emphasized  or  not  at  all. 

In  the  case  of  Pachuca,  the  bonanzas  are  irregular  or  roughly  elliptical  and 
are  not  shoot  like;  yet  the  fact  observed  by  Ordonez,  that  the  bonanzas  on  the 
different  veins  group  themselves  into  a  definite  zone  running  transversely  across 
the  strike,  is  hardly  to  be  accounted  for  except  by  the  explanation"  arrived  at 
in  the  case  of  Tonopah,  that  the  bonanzas  are  due  to  the  influence  of  an  intersecting 
fracture  system.  At  the  Comstock  the  bonanzas  are  similar  to  those  in  Pachuca, 
although  no  local  evidence  has  been  found  explaining  their  origin. 

The  above  explanation  is  readily  acceptable  for  bonanzas  that  are  elongated 
into  definite  shoots,  and  are  actually  known  to  be  associated  with  and  dependent 
upon  cross  fracturing,  as  in  Tonopah;  but  it  is  hot  so  easily  acceptable,  perhaps,  in 
the  case  of  wholly  irregular  bodies,  such  as  those  of  the  Comstock.  Yet  at  Tonopah 
the  bonanzas  are  irregularly  cut  off,  and  do  not  continue  indefinitely  downward 
on  the  pitch;  and  to  this  limitation  the  explanation  of  the  controlling  effect  of 
cross  fractures  must  unavoidably  be  extended.  Indeed,  an  inspection  of  the 
platting  of  fig.  24,  showing  the  principal  observed  faults  and  fractures  in  the 
Mizpah  mine,  and  a  reflection  that  this  is  diagrammatic,  while  the  real  fractures 
and  their  intersections  will  be  much  more  varied  and  localized,  shows  that  the 
intersections  of  such  mazes  (such  intersections  constituting  the  tortuous  channels 
of  most  active  circulation)  with  the  main  vein  fractures  will  often  be  quite  irreg- 
ular— will  only  approach  a  shoot-like  form  when  dominated  by  some  stronger  set 
of  cross  fracturing,  and  will  cease  to  produce  ore  bodies  or  bonanzas  of  definite 
form  when  there  is  no  controlling  fracturing,  and  now  one  fracture,  now  another, 
invites  and  controls  the  circulation. 

EXISTENCE   OF   A   MAJOR  PACIFIC  TERTIARY   PETROMETALLO- 

GRAPH1C  ZONE. 

Some  further  notes  may  be  added  to  the  above  references  (see  p.  275)  to  the 
extension  of  the  belt  of  late  Tertiary -Pleistocene  andesites. 

In  the  region  of  Krakatua  (situated  between  Sumatra  and  Java)  the  belt  of 
recent  and  active  volcanism  turns  eastward  and  passes  through  the  East  India 
Islands  and  adjoining  island  groups,  paralleling  the  Australian  coast,  then  curving 

a  Mr.  8.  F.  Emmons  informs  me,  on  reading  the  manuscript  of  this  report,  that  the  above  explanation  Imd  been 
adopted  at  l'iichm-H  when  he  was  there  in  1901. 


COMPARISON    WITH    SIMILAR    ORE    DEPOSITS    ELSEWHERE.  279 

southward  extends  through  New  Zealand.  Still  farther  southward  the  zone 
extends  through  the  Macquarie  Islands,  and  beyond  this,  in  antarctic  regions,  in 
Victoria  Land,  where  are  the  volcanic  cones  of  Erebus,  Terror.  Melbourne,  and 
Discovery,  of  which  one — Erebus — is  in  almost  continuous  eruption. 

The  prolongation  of  the  zone  goes  through  the  unexplored  antarctic  regions, 
very  near  to  the  south  pole,  and  on  the  other  side  there  are  Pleistocene  and 
recent  volcanoes  in  the  South  Shetland  Islands  and  other  near-by  land.  Not  far 
beyond  this  the  belt  comes  to  Tierra  del  Fuego,  a  desolate  volcanic  region. 
Thus  the  entire  circuit  of  the  earth  has  been  made.  This  girdle,  extending 
around  the  world  and  measuring  some  35.000  kilometers,  has  been  called  the 
"circle  of  fire"  by  geographers,  and  is  the  theater  of  the  world's  most  extensive 
and  active  volcanic  manifestations.  Within  this  circle,  in  the  Pacific  Ocean,  are 
lesser  volcanic  belts."  The  major  volcanic  belt,  when  viewed  on  a  globe  or  a 
perpendicularly  projected  map,6  has  not  a  circular  form,  but  rather  that  of  a 
great  somewhat  elongated  rectangle,  inscribed  upon  the  sphere;  the  two  longer 
sides  run  northwestward  and  consist  of  the  northwest  American  Pacific  coast  on 
one  side  and  the  stretch  from  the  Philippines  to  the  south  pole  on  the  other; 
the  two  shorter  sides  run  northeastward  and  consist  of  that  portion  lying  parallel  to 
the  Asiatic  coast  line  on  the  one  side  and  that  portion  in  and  near  the  antarctic 
regions  on  the  other.  This  figure,  however,  is  broken  by  irregularities  consisting 
of  curves  and  angles;  and  the  volcanic  chains  are  characteristically  arranged  in 
curves  or  "garlands,"''  though  in  many  cases  it  may  prove  true  that  such 
apparent  curves  are  in  reality  combinations  of  straight  lines,  as  is  the  case  with 
the  changes  of  trend  in  the  volcanoes  of  Java  and  Sumatra.  <' 

The  Pleistocene-Recent  volcanoes  of  the  East  Indies  belt,  which  began  their 
activity  toward  the  close  of  the  Tertiary,'  have  emitted  chiefly  andesites  with  a 
less  amount  of  closely  related  basalt.  Hornblende  or  pyroxene  andesite.  or  both, 
occur  in  Java,  Borneo,  Celebes,  and  neighboring  islands.  Most  of  the  pyroxene 
andesites  have  more  hypersthene  than  augite.-'" 

In  New  Zealand  hornblende-andesites  are  common.''  Concerning  the  recent 
lavas  of  the  Macquarie  Islands  and  other  antarctic  volcanic  regions,  there  appears 
to  be  little  information;  the  lava  of  Mount  Terror,  in  Victoria  Land,  is  reported  as 
"basic."* 

a  See  Reclus,  Elisee,  Nouvelle  geographic  universelle,  vol.  14.  pp.  41.  42:  Suess,  E.,  La  face  de  la  terre,  Paris,  vol.  2,  p. 
837;  Bonney,  Volcanoes,  London,  1899.  pp.  259-260:  Ferrar,  H.  T.,  Geog.  Jour.,  Apr.,  1905,  pp.  374,  et  seq. 
&  Reclus,  op.  cit.,  p.  43. 
c  Suess,  E.,  op.  cit.,  p.  339. 
<t  Bonney,  Volcanoes,  London,  1899,  p.  226. 
e  Zirkel,  Lehrbuoh  d.  Petrographie,  vol.  2,  p.  828. 
/Zirkel,  op.  cit.,  pp.  615,  616,  828,  829. 
orHutton,  F.  W.,  cited  by  Zirkel,  op.  cit.,  vol.  2,  p.  618. 
*  Ferrer,  H.  T.,  Geog.  Jonr.,  Apr.,  1905,  p.  375. 


280  GEOLOGY   OF   TONOPAH    MINING    DISTRICT,   NEVADA. 

There  appear,  then,  reasons  for  believing  that  the  belt  of  very  late  Pliocene- 
Pleistocene-Recent  andesitic  eruptions  continues  farther  than  suggested  in  the 
writer's  paper  quoted  above  (p.  275),  and  even  that  they  are  characteristic  of  the 
whole  great  "circle  of  fire;"  and  this  uniformity  seems  to  indicate  a  single 
major  petrographic  province  for  this  period,  extending  around  the  whole  zone." 

In  some  cases  the  analogy  of  the  less-known  Asian  and  Australasian  portions 
of  this  belt  with  the  North  American  part  is  known  to  extend  back  of  the  Pleis- 
tocene. In  the  East  Indian  archipelago,  according  to  Zirkel,  there  was  a  general 
eruption  of  pyroxene-andesite  at  the  end  of  the  Eocene  or  beginning  of  the 
Miocene,  since  the  early  Miocene  sediments  already  contain  some  andesitic  material. 
This  period  would  correspond  to  group  No.  2  of  the  scheme  of  succession  presented 
on  page  68.* 

In  New  Zealand  the  Hauraki  Peninsula  is  made  up  almost  wholly  of  Tertiary 
igneous  rocks,  mostly  andesites,  with  accompanying  heavy  deposits  of  volcanic 
agglomerates;  these  andesites  and  accompanying  tufl's  and  breccias  are  regarded 
as  of  late  Eocene  and  early  Miocene  age.  In  places  they  are  covered  by  rhyo- 
lites  and  rhyolitic  tuffs  of  early  Pliocene  age.c  These  andesites  and  rhyolites, 
respectively,  fall  into  groups  2  and  3  of  the  scheme  on  page  68. 

It  is  also  probable  that  the  coextension  of  the  metallographic  and  the  petro- 
graphic provinces  is  greater  than  above  established,  for  at  many  other  points  along 
the  belt  of  the  petrographic  province,  in  the  Andes  of  South  America  (for  example, 
in  Peru*),  veins  are  reported  having,  so  far  as  can  be  made  out,  a  mode  of  occur- 
rence, age,  and  composition  similar  to  those  of  Mexico.  The  mines  at  Quespasia  in 
that  country  are  in  highly  altered  augite-andesite.  The  ore  minerals  are  pyrargy- 
rite,  polybasite,  and  other  rich  silver  ores,  with  galena  and  blende,  and  a  little 
copper  pyrite  and  iron  pyrite.  In  their  richest  portions  they  contained  on  an 
average  2  per  cent  silver/  These  richest  portions  in  the  Peruvian  mines  of  this 
type  are  like  the  Mexican  bonanzas,  and  are  called,  in  Peru,  tajos.f 

At  Cerro  de  Pasco,  also  in  Peru,  the  argentiferous  formation  is  a  metamor- 
phosed Mesozoic  sandstone  intruded  by  altered  andesite.  The  ore  consists  of 
free  silver,  silver  sulphides  and  antimonides,  lead  carbonate  and  sulphide,  various 

oThese  andesites.  constituting  the  most  recent  lava  of  this  province,  appear  to  be  a  distinctly  later  group  in  the 
volcanic  succession  than  the  youngest  (No.  6)  enumerated  in  the  scheme  on  p.  68.  They  may  be  designated  as  group 
No.  6,  Pleistocene  and  Recent,  and  the  recurrence  of  lava  of  this  composition,  similar  to  Nos.  2  and  4  (early  Miocene  and 
late  Pliocene  andesites,  respectively), suggests  the  beginning  of  a  new  cycle  of  magmatic  differentiation.  \\  host-  continua- 
tlnn  will  bring  about,  for  the  fourth  time  In  the  history  of  this  volcanic  epoch,  the  eruption  of  basalts  and  rhyolites 
similar  to  Xos.  1,  3,  and  5.  (See  Spurr,  J.  E.,  Jour.  Geol.,  vol.  8,  No.  7.  pp.  637-646.) 

In  the  region  near  Tonopah  there  is  one  probable  representative  of  these  latest  andesites.  In  Mono  Lake,  Cali- 
fornia, 90  miles  west  of  Tonopah,  are  ten  or  fifteen  volcanic  cones  of  very  recent  date,  the  lavas  being  in  part  hypersthene- 
andeHite.  In  part  rhyolltc.  (Russell,  I.  C.,  Eighth  Ann.  Rept.  U.  S.  Geol.  Survey,  pp.  374,  375,  377,  380.) 

6  See  Spurr,  J.  E.,  Jour.  Cieol.,  vol.  8,  No.  7,  p.  637. 

••  Park,  James,  elted  by  Lindgren,  W.,  Eng.  and  Min.  Jour.,  Feb.  2,  1906,  p.  218. 

•'  FuchH  ct  de  Launay,  Gltes  metal  11  feres,  vol.  2,  p.  829. 

•  Beck,  Erzlagerstfitten,  2d  ed.,  p.  277. 

/  Fucha  et  de  Launay,  op.  cit.,  vol.  2,  p.  831. 


COMPARISON    WITH    SIMILAR    ORE    DEPOSITS    ELSEWHERE.  281 

copper  minerals,  zinc,  and  iron  pyrite.  Twenty-seven  miles  from  Cerro  de  Pasco 
are  veins  in  quartz-porphyry  (rhyolite?).  The  ore  contains,  besides  silver  min- 
erals, various  copper  minerals,  galena,  sphalerite,  bismuthinite,  and  stibnite." 

In  view  of  the  presence  of  selenium  at  Tonopah,  the  occurrence  of  this 
element  at  other  places  along  this  Pacific  petrographic  province  in  America  is 
of  interest.  At  Guanajuato,  northwest  of  the  city  of  Mexico,  selenides,  including 
a  sulpho-selenide  of  silver,  occur  in  argentiferous  veins  in  hornblende  andesite.6 
At  Tasco,  180  miles  southeast  of  Guanajuato,  crystallized  selenide  of  silver 
occurs/  In  the  South  American  Andes  selenides  occur  at  the  Cacheuta  silver  mine, 
province  of  Mendoza,  Argentina,  whose  vein  is  in  "trachyte."''  They  include  the 
selenide  of  lead  and  copper,  that  of  copper  and  silver,  and  others.  The  latter  selenide 
occurs  also  in  the  Chilean  Andes,  at  Copiapo  and  Flamenco,  and  elsewhere/ 

It  is  also  interesting,  in  regard  to  the  speculations  of  the  author  above  quoted 
concerning  the  Asiatic  prolongation  of  the  petrographic  province,  to  note  that  in 
Japan  veins  of  argentiferous  quartz  are  being  worked,  which  occur  in  the  midst  of 
Tertiary  eruptives,  and  which  belong  to  the  Comstock  type.-''  Explicit  information 
concerning  these  has  lately  come  to  hand.*  Tertian*  and  Quaternary  volcanic  rocks 
are  widely  distributed  in  northern  Japan.  The  Tertiary  rocks  include  rhyolite 
(as  old  as  the  beginning  of  the  Tertiary),  andesite,  and  basalt.  Metalliferous  veins 
in  Tertiary  andesite  and  rhyolite  are  among  the  most  important  mineral  resources 
in  Japan.  The  older  andesites  have  often  suffered  alteration  by  mineral  waters 
and  gases. 

The  Hoshino  mines,  in  Hoshino-mura,  Chikugo  province,  are  in  augite-andesite. 
The  deposits  are  quartz  veins  containing  pyrite,  blende,  gold,  and  silver.  The 
Serigano  mine,  in  Satsuma  province,  is  in  augite-andesite;  the  gangue  is  quartz,  and 
the  metallic  minerals  are  pyrite,  chalcopyrite,  gold,  and  silver.  The  Yamagano 
district,  between  Satsuma  and  Osuini,  is  at  present  the  most  promising  in  the 
country.  Here  are  numerous  veins  in  augite-andesite.  The  gangue  is  quartz,  often 
containing  calcite  and  pyrite.  The  ore  is  native  gold  associated  with  argentite,  and 
rarely  with  chalcopyrite.  The  proportion  of  gold  to  silver  is  about  5  to  1.  At  the 
Ponshikaribets  mine,  Shiribeshi  province,  the  country  rocks  are  Tertiary  tuffs,  cut 
by  andesite  dikes.  The  gangue  is  rhodochrosite  and  quartz,  the  ores  are  auriferous 
argentite,  galena,  chalcopyrite,  and  blende.  The  mine  of  Aikawa,  in  Sado  province, 
has  had  an  enormous  production.  The  veins  are  in  augite-andesite  and  Tertiary 

o  Mason,  Russell  T.,  Eng.  and  Min.  Jour.,  June  8,  1905,  p.  1092. 

fcTrans.  Am.  Inst.  Min.  Eng.,  vol.  32,  p.  501.    Dana,  System  of  Mineralogy,  6th  ed.,  p.  1025. 

cDana,  op.  cit.,  p.  52. 

<*Fuchs  et  de  Launay,  GHes  m^talliferes,  vol.  2,  p.  832.    The  "trachyte"  is  probably  andesite. 

«Dana,  op.  cit.,  pp.  53,  54.  . 

/  Fuchs  et  de  Launay,  op.  cit.,  p.  832. 

a  Geology  of  Japan,  Geol.  Survey,  Tokyo,  1902,  pp.  18, 19,  118, 124-171. 


282  GEOLOGY    OF   TONOPAH    MINING    DISTRICT,    NEVADA. 

tuff*.  The  gangue  is  quartz,  with  calcite,  rarely  with  dolomite  and  gypsum. 
The  ores  are  chiefly  native  gold  and  silver,  and  argentite,  associated  with  chalco- 
pyrite,  pyrite,  blende,  and  galena;  rarely  stephanite,  pyrargyrite,  marcasite,  and 
arsenopyrite.  At  the  Kosen  mine,  in  Tajima  province,  the  veins  are  connected  with 
"propylite''  dikes  in  granite.  The  gangue  is  quartz,  the  ore  auriferous  argentite, 
with  pyrite  and  galena.  The  Tasei  mine,  Tajima  province,  is  in  "propylite," 
rhyolite,  and  Tertiary  tuffs.  The  gangue  of  the  vein  is  quartz,  with  some  calcite 
and  rhodochrosite.  The  ores  are  argentite  and  native  gold  and  silver,  with 
chalcopyrite,  pyrite,  galena,  blende,  and  malachite.  At  the  Kanagase  mine,  not 
far  distant,  the  country  rocks  are  similar;  the  gangue  is  quartz  and  calcite,  and 
the  ores  are  chalcopyrite,  bornite,  pyrite.  tetrahedrite,  argentite,  galena,  stibnite, 
pyrargyrite,  blende,  bismuth,  and  native  silver  and  copper.  At  the  Omori  mine, 
Iwami  province,  the  rocks  are  bypersthene-quartz-andesite,  andesite  agglomerate, 
and  Tertiary  strata.  The  ores  are  in  veins  and  impregnation  deposits.  The  gangue 
is  quartz;  the  ore  native  silver,  argentite,  siderite,  malachite,  and  auriferous  and 
argentiferous  chalcop3rrite.  The  Okuzu  mine,  in  Ugo  province,  is  in  Tertiary 
tuff  and  augite-andesite.  The  gangue  is  quartz:  the  ore  auriferous  chalcopyrite, 
with  pyrite  and  rare  blende.  Silver  is  rare.  At  the  Mizusawa  mine,  Ugo  province, 
the  country  rock  is  augite-andesite  and  Tertiary  strata.  The  ore  is  a  mixture  of 
barite.  argentite,  blende,  galena,  pyrite,  quartz,  calcite,  chalcopyrite,  and  probably 
stephanite.  At  the  Tsubaki  and  Hachimori  mines,  Ugo  province,  veins  in  andesite 
carry  ores  like  the  last  named.  At  the  Shirayama  mine,  Ugo  province,  veins  in 
Tertiary  tuff  and  augite-andesite  have  a  gangue  of  quartz  and  barite,  and  contain 
argentiferous  "galena,  blende,  pyrite,  and  chalcopyrite.  At  the  Innai  mine,  Ugo 
province,  the  country  rock  is  Tertiary  "propylite."  the  gangue  is  quartz  and 
rhodochrosite,  the  ore  minerals  stephanite,  argentite,  pyrargyrite,  chalcopyrite, 
pyrite,  galena,  and  blende.  At  the  Towada  mine,  in  Rikuchu  province,  the  vein 
occurs  in  Tertiary  tuff,  associated  with  augite-andesite.  The  ore  is  auriferous 
argentite  and  chalcopyrite  in  a  clay  and  gypsum  matrix.  At  the  Omaki  mine, 
Ugo  province,  the  country  rocks  are  Tertiary  tuffs  and  andesite.  The  ore  is 
argentite,  silver  oxide,  copper  and  iron  pyrite,  and  galena,  with  barite  and  g\'psum 
as  gangue  minerals.  At  the  Hisanichi  mine,  Ugo  province,  is  a  vein  in  Tertiary 
strata  and  augite-andesite.  The  ore  is  galena,  chalcopyrite,  blende,  and  pyrite. 
Many  of  the  important  metalliferous  veins  in  northern  Japan  and  Chugoku 
are  also  in  rhyolites.  In  the  Kanahira  mine,  in  Kananomura,  Kaga  province,  the 
veins  are  in  rhyolite;  the  gangue  is  barite  and  quartz,  the  ores  are  native  gold, 
blende,  and  pyrite.  At  the  Matsuoka  mine,  in  Ugo  province,  the  ore  is  a  stockwork 
at  the  contact  of  rhyolite  with  Tertiary  strata;  the  ores  are  argentiferous  galena, 
blende,  and  pyrite,  carrying  gold.  At  the  Handa  mine,  Iwashiro  province,  the 


COMPARISON    WITH    SIMILAR    ORE    DEPOSITS    ELSEWHERE.  283 

veins  are  in  rhyolite  and  Tertiary  strata.  The  gangue  is  quartz  with  calcite  and 
amethyst;  the  ore  is  auriferous  argentite,  with  blende;  galena,  pyrite,  and  native, 
silver  are  sometimes  found.  At  the  Takadama  mine,  Iwashiro  province,  quartz 
veins  containing  auriferous  argentite  occur  in  rhyolite  and  Tertiary  strata.  The 
Kuratani  mine,  in  Kaga  province,  contains  veins  in  rhyolite  and  Tertiary  tuffs. 
The  gangue  is  rhodochrosite,  with  barite  and  calcite;  the  ores  contain  argentiferous 
galena,  blende,  pyrite,  and  jamesonite,  and  carry  gold.  At  the  Tagonai  mine,  Ugo 
province,  the  veins  are  in  Tertiary  tuff,  augite  andesite,  and  rhyolite;  the  gangue 
minerals  are  quartz  and  barite,  the  ores  argentiferous  galena,  blende,  and  pyrite. 
At  the  Hata  mine,  Ugo  province,  the  rocks  are  Tertiary  tutf  and  rhyolite;  gangue 
minerals  are  quartz,  calcite,  and  barite;  the  ores  are  argentite,  galena,  pyrite,  and 
chalcopj'rite.  At  the  Kuromori  mine,  Iwaki  province,  the  vein  is  in  rhyolite. 
The  gangue  is  quartz,  often  amethystine;  the  ore  is  argentite,  with  blende.  At 
the  Kosaka  mine,  in  Rikuchu  province,  the  ore  is  an  impregnation  in  Tertiary  tutf, 
with  rhyolite  and  dacite  intrusions;  it  consists  of  lead  and  copper  carbonates, 
copper  sulphate,  native  copper  and  silver,  and  barite.  At  the  Hatasa  mine,  Mino 
province,  the  rocks  are  rhyolite  (quartz-porphyry)  and  andesite.  The  veins  consist 
of  quartz  containing  argentiferous  chalcopyi'ite,  galena,  argentite,  blende,  and 
pyrite.  The  Waidani  mine,  Bizen  province,  is  in  rhyolite;  the  ores  are  argen- 
tiferous chalcopyrite,  blende,  and  galena. 

Besides  the  examples  above  cited,  other  veins  of  closely  related  types,  but 
often  containing  a  larger  amount  of  the  baser  ores  (lead,  zinc,  and  copper)  than 
the  more  abundant  cases  above,  occur  in  or  near  Tertiary  andesite  or  rhvolite. 

Some  information  is  available  concerning  certain  East  Indian  ore  deposits  on 
islands  lying  south  of  Japan  along  the  belt  characterized  by  similar  Tertiary  and 
Pleistocene  volcanics.  In  the  whole  of  the  Dutch  East  Indies,  according  to  S.  J. 
Truscott,"  the  gold  (which  is  always  accompanied  by  a  larger  amount  of  silver) 
occurs  in  reefs,  veins,  and  impregnation  zones,  in  altered  andesite  (porphyrite), 
or  near  the  contact  of  such  a  rock  with  Devonian  slates,  in  which  slates  there 
are  sometimes  similar  though  less  extensive  occurrences.  The  ore  deposition 
probably  took  place  in  the  Tertiary. 

One  of  the  principal  productive  centers  in  this  region  is  the  mine  Redjang 
Lebong,  in  the  southwest  part  of  Sumatra.  Here  the  ore,  which  occurs  in  altered 
andesite,  has  a  gangue  of  fine-grained  silica,  with  often  some  calcite.  The  gold  is 
finely  disseminated  and  is  rarely  visible;  it  exists  free  and  in  combination  with 
silver,  in  the  proportion  of  1  to  10.  At  depth  this  silver  probably  exists  as 
sulphide,  connected  with  pyrites.  Bullion  from  this  mine  gives  the  following 
analysis:  Gold  and  silver,  91.52  per  cent;  selenium,  4.35;  copper,  1.82;  lead,  1.65; 
zinc,  0.48;  iron,  0.14;  total,  99.96.  Tellurium  was  not  found. 

a  Trans.  Inst.  Min.  Metal.,  vol.  10,  pp.  52-73. 


284  GEOLOGY    OF   TONOPAH   MINING    DISTRICT,  NEVADA. 

The  similarity  of  Redjang  Lebong  to  Tonopah  has  been  commented  upon  by 
Mr.  Percy  Morgan,"  judging  from  the  writer's  earlier  description  of  Tonopah b 
and  from  reports  concerning  Redjang  Lebong.  This  similarity  was  also  called 
to  the  writer's  attention  by  Mr.  L.  Hundeshagen,  who  has  personally  visited  both 
districts.  The  discovery  of  selenium  in  the  Tonopah  ores,  in  somewhat  the  same 
proportion  as  indicated  in  the  above  analysis,  subsequent  to  the  comparisons 
made  by  these  gentlemen,  strikingly  strengthens  the  resemblance. 

Five  miles  west  of  Redjang  Lebong  is  a  similar  occurrence  of  gold  ore  in 
altered  andesite,  at  Lebong  Soelit. 

In  southeastern  Borneo  gold  occurs  in  altered  andesite/ 

The  northern  arm  of  Celebes  is  gold  bearing.  The  mine  at  Palehleh  is  in 
altered  andesite,  often  having  a  dioritic  aspect.  The  ore  contains  pyrite,  galena, 
zinc-blende,  and  copper  pyrite,  with  a  little  antimony  and  arsenic,  and  carries 
gold  and  silver,  of  which  the  sulphides  contain  gold  about  4£  ounces  and  silver 
12  ounces  to  the  ton/'  Forty  miles  west  of  Palehleh,  at  Soemalata,  the  ore  is  in 
andesite  or  uporphyrite."f  The  ore  is  like  that  at  Palehleh — heavy  sulphides  with 
some  quartz  gangue,  more  often  feldspar.  Ten  miles  west  of  Palehleh,  at  Denuki 
Bay,  are  ores  similar  to  those  at  Soemalata,  but  containing  more  quartz,  in  altered 
andesite.  Analysis  of  the  sulphides  shows  zinc,  31  per  cent;  lead,  8  per  cent; 
copper,  1  per  cent;  gold,  5.3  pennyweights  to  the  ton;  silver,  4.9  ounces  to  the 
ton;  arsenic,  2  to  4  per  cent;  antimony,  4  to  6  per  cent.  On  the  south  coast  of 
the  peninsula,  at  Totok,  are  heavy  auriferous  quartz  veins  in  altered  andesite; 
also  6  miles  southwest  of  Totok,  at  Kataboenan,  where  the  andesite  has  been 
intensely  silicified  on  each  side  of  a  central  fracture,  forming  a  wide  mass  of  ore 
of  the  following  average  composition:  Gold,  4  pennyweights  per  ton;  silver,  1 
ounce  per  ton;  sulphides,  6  per  cent;  vein  quartz,  3  per  cent;  the  remainder 
being  altered  andesite. 

Still  farther  along  the  Tertiary-Pleistocene  volcanic  zone  lies  New  Zealand. 
The  late  Eocene-early  Miocene  andesites  of  the  Hauraki  Peninsula,  in  the  north 
island  of  New  Zealand,  contain  throughout  veins  bearing  gold  and  silver.  The 
whole  peninsula  has  produced  $50,000,000.  Near  the  veins  the  'andesite  has  been 
altered  to  calcite,  chlorite,  serpentine,  quartz,  and  pyrite.  The  ore  in  the  Thames 
district  is  chiefly  native  gold  alloyed  with  30  to  40  per  cent  silver.  Associated 
minerals  are  dolomite,  pyrite,  chalcopyrite,  zinc- blende,  galena,  stibnite  and  ruby 
silver,  arsenopyrite,  and  native  arsenic/  Great  masses  of  quartz  are  very  low 
grade,  but  bonanzas  of  very  rich  ore  occur  at  the  intersection  of  feeders  with  the 
main  vein. 


a  Eng.  and  Min.  Jour.,  May  4,  1905,  p.  862.         rtTruscott,  loc.  cit.,  pp.  66-67. 

t>  Ibid.,  May  2, 1903.  eTruscott,  loc.  cit.,  p.  68;  also  Suess,  E.,  I>a  face  do  la  terre,  vol.  8,  p.  341. 

oTriuicott,  8.  J.,  los.  cit.,  p.  63.  /Lfndgren,  W.,  Eng.  and  M!n.  Jour.,  Feb.  2,  1905,  p.  218. 


COMPARISON    WITH    SIMILAR    ORE    DEPOSITS    ELSEWHERE.  285 

At  Karangahake  the  ore  is  argentite,  with  a  little  pyrite  and  free  gold,  in 
drusy,  fine-grained  quartz;  stibnite  and  calcite,  with  some  siderite  and  a  little 
nickel  and  cobalt,  also  occur.  At  the  Waihi  mine,  which  up  to  the  end  of  1903 
had  produced  $15,000,000,  the  ores  are  in  altered  andesitic  rock,  and  have  been 
covered  by  later  rhyolitic  flows.  The  oxidized  quartz  contains  argentite  and  free 
gold,  with  black  oxide  of  manganese,  and  oxides  of  nickel  and  cobalt;  the  sulphide 
ores  contain  pyrite  and  blende,  with  a  little  nickel,  cobalt,  and  selenium.  The 
country  rock  is  altered,  the  secondary  products  including  pyrite,  carbonate  (calcite?), 
and  serpentine.  In  the  veins  and  veinlets  the  gangue  minerals  are  quartz,  calcite, 
and  adularia. 

There  are  two  distinct  flows  of  rhyolite  overlying  the  andesite,  of  which  the 
older  has  a  remarkable  flow  structure,  giving  it  a  brecciated  appearance."  There 
has  been  a  later  period  of  mineralization,  producing  gold-bearing  lodes  in  rhyolite.* 

Mr.  Lindgren  calls  attention  to  the  striking  similarity  between  the  Waihi  mine 
and  the  De  Lamar  mine,  in  Idaho,  described  by  him — a  mine  already  likened  to 
Tonopah  by  the  writer  (see  p.  271).  Mr.  Morgan,  judging  from  a  personal  knowl- 
edge of  Waihi  and  the  writer's  description  of  Tonopah, c  calls  attention  to  the  close 
resemblance  of  these  two  districts. 

Tellurium  occurs  in  some  of  the  New  Zealand  districts,  varying  from  traces 
up  to  12  ounces  per  ton,  in  picked  samples.  Samples  from  various  districts  show 
the  following  types  of  ores  in  regard  to  gold,  silver,  and  tellurium:  Coromandel, 
25  per  cent  mispickel,  gold  200  ounces,  silver  90  ounces,  a  little  tellurium;  Tapu, 
2i  ounces  gold,  250  ounces  silver,  7£  ounces  tellurium;  Waiomo,  gold  15  ounces, 
silver  600  ounces,  tellurium  12  ounces;  Waiomo,  Monawai,  gold  2  ounces,  silver 
40  ounces,  tellurium  4  ounces.  No  tellurium  was  detected  in  samples  from 
Waihi,  Jubilee,  Komata,  Karangahake,  and  Great  Barrier,  in  which  the  gold 
and  silver  bore  the  following  proportions:  Gold  24  ounces,  silver  760  ounces; 
gold  8  ounces,  silver  150  ounces;  gold  600  ounces,  silver  160  ounces;  gold  2 
ounces,  silver  256  ounces;  and  gold  2  ounces,  silver  200  ounces.  Thus  tellurium 
has  been  found  in  a  line  stretching  from  Coromandel  to  Maratoto,  but  nowhere 
to  the  east.rf 

Besides  the  selenium  noted  above  in  the  Waihi  mine,  Mr.  Allen  found  selenium 
in  the  ore  at  Great  Barrier.  In  this  New  Zealand  region  selenium  and  tellurium 
have  not  been  proved  to  be  present  in  the  same  district.  This  is  especially 
interesting  in  comparing  New  Zealand  with  the  Nevada  region,  where  selenium 

"Compare  the  description  of  the  Tonopah  rhyolite  dacite,  p.  41. 

ftLindgren,  ut  supra;  also  Morgan.  Percy,  Eng.  and  Min.  Jour.,  May  4, 1905;  Trans.  Austral.  Inst.  Min.  Eng.,  pp.  1&1-187. 

cEng.  and  Min.  Jour.,  May  2,  1903. 

d Allen,  F.  B.,  Trans.  Austral.  Inst.  Min.  Eng.,  vol.  7,  p.  M. 


286  GEOLOGY    OF    TONOPAH    MINING    DISTRICT,   NEVADA. 

without  tellurium  has  been  found  at  Tonopah,  and  tellurium  without  selenium  at 
Goldfield,  28  miles  south. 

Enough  data  has  been  given  above  to  indicate  the  coordination  of  an  inter- 
esting set  of  phenomena.  The  greatest  of  the  earth's  oceans  is  rimmed  by  the 
greatest  of  the  earth's  volcanic  belts.  This  "circle  of  fire,"  whether  it  runs  along 
the  coast  of  the  mainland,  as  in  the  Americas,  or  along  chains  of  islands,  as  in 
the  Asian  and  Australian  regions,  follows  faithfully  the  Pacific-fronting  outlines 
of  the  continents  of  South  America,  North  America,  Asia,  and  Australia,  and 
demarks  the  continental  from  the  oceanic  areas.  In  the  Asian,  Australasian,  and 
Australian  regions,  indeed,  the  outlying  islands  rather  than  the  continents  have 
been  held,  from  a  geological  viewpoint,  to  represent  the  limits  of  the  Pacific 
Ocean. a  Topographically  the  volcanic  belt  is  also  marked  throughout  its  course 
by  a  line  of  bold  and  towering  mountains,  the  consequence  of  active  and  com- 
paratively recent  extravasation  and  uplift. 

For  the  next  step  in  coordination  the  data  are  not  so  complete,  but  our  informa- 
tion goes  to  show  that  remarkably  similar  lavas  have  been  erupted  from  the  active 
and  recently  extinct  cones  which  are  ranged  along  this  belt. 

A  still  smaller  fund  of  information  is  available  for  the  next  step,  but  we  are 
led  to  it  by  all  that  we  can  learn.  It  is  that  the  ''circle  of  fire"  existed  as  such 
throughout  most  of  the  Tertiary,  and,  moreover,  that  the  similarity  of  the  more 
recent  lavas  was  paralleled  by  like  similarities  at  the  different  earlier  stages  of 
eruption.  Roughly  speaking,  the  idea  is  suggested  that  throughout  the  zone  the 
order,  period,  and  nature  of  the  different  erupted  lavas  have  been  approximately 
the  same. 

This  belt  also  contains  an  extraordinary  number  of  extraordinarily  rich  silver- 
gold  ores  (as  well  as  those  of  lead,  copper,  zinc,  etc.).  These  ores  are  contained  in 
or  associated  with  Tertiary  andesites  and  to  a  less  extent  rhyolites  (chiefly  Miocene 
andesites  and  Pliocene  rhyolites);  and  wherever  they  occur  the  nature  and  propor- 
tion of  the  ore  and  gangue  minerals  and  the  nature  of  alteration  of  the  country 
rock  are  uniform  to  a  surprising  degree.6  Similar  mineralizing  solutions,  dependent 
upon  the  eruption  of  similar  lavas  at  the  same  geological  period,  are  attested. 

The  significance  of  the  geographic  coincidence  of  these  different  phenomena, 
occurring  on  so  stupendous  a  scale  as  to  stand  out  unmistakably  from  the  confusion 
of  detail  of  the  world's  geology,  has  yet  to  be  thoroughly  understood.  These 
geographically  coinciding  phenomena  may  be  summed  up  as  follows: 

1.  The  borders  of  the  earth's  greatest  ocean. 

2.  The  most  persistent  of  the  earth's  lofty  and  bold  mountain  belts. 

aVon  Drasche,  P.,  cited  by  guess,  E.,  La  face  de  la  terre,  Paris,  vol.  2,  p.  339. 

&At  and  near  Schemnltz,  in  Hungary,  ore  veins  and  ore  similar  to  those  of  this  great  Pacific  province,  and  they 
occur  under  simiar  geologic  conditions.  Otherwise  no  good  example  outside  of  the  province  has  come  to  the  writer's 
notice. 


COMPARISON    WITH    SIMILAR    ORE    DEPOSITS    ELSEWHERE.  287 

3.  The  belt  of  the  earth's  most  active  and  extensive  recent  vulcanism. 

4.  A  belt  showing  .similar  recently  erupted  lavas. 

5.  A  belt  showing  similar  lavas  erupted  during  the  Tertiary. 

6.  A  belt   of   enormous   and   roughly   uniform   later  Tertiary   mineralization, 
involving  great  concentration  of  silver  and  gold. 

When  it  is  considered  that  solutions  accompanying  (and  presumably  emanating 
from)  Miocene  andesites  (to  a  less  extent  Miocene-Pliocene  rhyolites)  in  this 
particular  restricted  zone  have  produced  a  very  large  proportion  of  the  world's 
available  supply  of  the  precious  metals,  the  rare  and  special  nature  of  the  occur- 
rences which  have  called  these  ore  deposits  into  being  becomes  evident,  and  it 
becomes  impossible  to  entertain  any  explanation  based  upon  processes  uniformly 
distributed  throughout  the  world. 


INDEX. 


A. 

Page. 

Adularia,  alteration  of  earlier  andesite  to 207-209 

analysis  of 87 

character  of 86-N7, 228 

formation  of,  chemistry  of 230-231 

occurrence  of 22, 229-230 

explanation  of 212-218 

origin  of 22,228,231 

sericite  and,  relations  of 227-228 

view  of 208 

Aguilera,  J.  G.,  on  Mexican  ores 269-270  | 

Albite,  occurrence  of,  explanation  of 212-213 

Allen,  E.  T.,  analysis  by 252  ] 

Alteration.    See  Rock  alteration. 

Ancylus?  sp.,  occurrence  of 67 

Andesite,  earlier,  alterations  of 22, 31-32, 207-238 

alterations  of,  diagrams  showing 218, 234, 218 

analyses  of 57, 64, 65, 216 

character  of 21,31-32,63,164-165,253 

classes  of 32 

later  andesite  and,  comparison  of 35 

occurrence  of 21, 32, 101, 164-165, 186-187 

specimens  of,  description  of 213-216 

veins  in 22, 83-% 

age  of 83 

circulation  channels  in 831 

cross  walls  in 85 

mineralization  of 85 

minerals  ^^ 90 

view^I 208 

mines  ancrwospects  in,  description  of 115-191 

ores  of  ...." 86-90 

origin  of 84-85 

oxidation  of 90-94 

Andesite,  later,  alteration  of 23, 238-252, 254 

alteration  of,  figures  showing 242. 243 

period  of 250-252 

analyses  of 57, 66 

character  of 21,33-34,253 

diagrams  showing 242 

earlier  andesite  and,  comparison  of 35 

occurrence  of 21, 34-35, 101, 179 

origin  of 22 

shafts  in ; 205-206 

study  of 245-248 

veins  in 261 

Ararat  Mountain,  erosion  on 113 

rocks  in 30,36,49,55,101,191-192,253-254 

veins  in 22,101-104 

location  of,  map  showing 102 

Argentite,  occurrence  of 22,92,94-95 

Arid  lands,  erosion  in 111-112 

16843— No.  42—05 19 


Page. 

Bagg,  R.  M.,  jr.,  fossils  determined  by 69 

Barium,  alteration  of 233-234 

Basalt,  age  of 56 

analysis  of 57.64,65 

character  of 21, 56, 63 

occurrence  of 45, 55 

origin  of 21-22 

Becker,  G.  F..  on  mineralizing  waters 237 

on  propylite 236 

on  sideritc 249 

on  the  Comstock  lode 259,265 

Belle  of  Tonopah  shaft,  location  of 198 

rocks  and  veins  in 198-199 

water  in 106 

Belmont  Company,  property  of 125 

Belmont  shaft,  depth  of 106, 19S 

lava  from,  analysis  of 57, 60, 64 

location  of 193 

rocks  in 30,61.193-194 

section  near,  figure  showing 193 

Big  Tono  shaft,  data  concerning 200 

depth  of 106 

Biotite,  alteration  of 207,232 

Bischof ,  Gustav,  on  feldspar  alteration 212-213, 233 

Blende,  character  of 88 

occurrence  of 22 

Bonney,  T.  G.,  on  volcanoes 260 

Booker  &  Bradford,  map  prepared  by 26 

Borneo,  rocks  of 284 

Boston  Tonopah  shaft,  depth  of 106, 192 

location  of 192 

rocks  in 101,192 

Brauns,  D.,  on  mnscovite 281 

Brogger,  W.  C.,  diagram  by 242 

Brougher,  W.,  mention  of 26 

Brougher  dacite,  analysis  of 60, 65 

areas  of,  diagram  showing 50 

character  of 44-48,62-63 

contacts  of,  shafts  at 200 

faulting  due  to 47 

hornblendes  in 62 

occurrence  of 44 

origin  of 46-47 

Siebert  tuffs  and,  relations  of,  figure  showing 53 

Brougher  Mountain,  character  of 44 

erosion  on 113 

faulting  near :  75, 146 

history  of 67 

lava  of,  analysis  of 57, 58, 60, 65 

origin  of .' 146 

rocks  of 36,46,49-51,55,60-61 

289 


290 


INDEX. 


Page. 

Brougher  Mountain,  section  near,  diagram  showing..  41,43 
view  of 46 

Brougher  shaft,  section  in,  figure  showing 116 

Burro  fault,  discovery  of 79 

Burro  veins,  data  concerning 127-129 

structure  of,  section  showing 128 

Butler.  Jas.  L.,  gold  discovered  by 21,25-20 

leases  by 26 

on  Monitor  Valley  springs 257 

Butler  Mountain,  character  of 44, 110-111 

elevation  of 25 

erosion  on 113 

eruptions  on 67 

lava  from,  analysis  of 58, 60, 65-66 

rocks  of 36,45,55,60-61 

section  near,  diagram  showing 44, 48, 49 

view  from 24, 44 

view  of 44, 46 

Butte  Tonopah  shaft,  data  concerning 199 

C. 

Calcite,  alteration  of  earlier  andcsite  to 210 

formation  of 22, 229 

veins  of 101-104 

California  fault,  location  of 167 

rocks  at 38,79 

Cambrian  rocks,  occurrence  of 30 

Cavities,  filling  of,  veins  formed  by 85 

Celebes,  rocks  of 284 

Chalcopyrite,  character  of 88 

occurrence  of 22, 95 

Chlorite,  alteration  of  earlier  and esite  to 22,210 

Cinder  cones,  occurrence  of 45 

Circle  of  fire,  location  of 279 

ores  of 286 

rocks  of 279-280, 286 

Circulation  of  water,  channels  of 85 

channels  of,  alteration  in 210 

Claims.    See  Mines. 

Coal,  character  and  occurrence  of 29 

Comstock,  Nev.,  alteration  at 211-212 

ores  of  Tonopah  and,  comparison  of 22, 

270-271,273-274.278 

section  at,  figure  showing 277 

underground  temperatures  at 265-266 

figure  showi  ng 265 

Contacts,  phenomena  of 44, 49, 79 

Corbicula  occidentalis.  occurrence  of 67 

Coscinodiscus  radiatus,  occurrence  of 70 

Crislo  vein,  section  of,  figure  showing 277 

Cross  faulting.    .See Faulting,  cross. 

Cross  walls,  effects  of 85 

figure  showing 173 

occurrence  of 119 

D. 

Dacite,  analyses  of 57, 58, 60, 65,  i>6 

character  of 21, 36-51,  S8-61 

classification  of 59-60 

contact  of,  section  showing 48 

fault  blocks  and,  displacement  of,  diagram  show- 
ing        46 

occurrence  of 21,36-51 

origin  of 22, 63 

sections  of,  diagrams  showing 40, 41, 44, 45, 48 

shattsin 200-204 

Siebert  tuff  and,  contact  of,  figure  showing 45 

MI- w  nl 46,48 


Page. 

Dall,  W.  H.,  fossils  determined  by 66 

Dana,  J.  D.,  on  pyroxene 248 

Daubree,  A.,  on  underground  water 254 

De  Lamar,  Idaho,  ores  of 276-277, 285 

ores  of  Tonopah  and,  comparison  of 22, 271-274, 277 

section  at,  figure  showing 277 

Depth,  temperature  and,  relations  of.    See  Temperature. 

Desert  Queen  shaft,  depth  of 197 

description  of 125-127, 193 

r°eks  in 125-12?!  193 

section  through,  figure  showing 177 

values  in 97 

veins  in,  plan  showing 13 

water  in 105, 125 

workings  at.  plan  of 126 

Devils  Punchbowl,  hot  springs  of 257 

Differentiation  theory  of  origin  of  lavas 61-66 

tests  of,  by  analysis 64, 65, 60 

Dikes,  occurrence  of 44-45, 73-74 

section  showing 49 

Doelter,  C.,  on  museovite 231 

Dominian,  Leon,  measurements  by 263 

E. 

East  Indies,  rocks  of 279-280, 283 

Eocene  rocks,  fossils  from 66-67 

Eocene  time,  history  in 66, 69 

Epidote,  occurrence  of 250 

Erosion,  amount  of 1 10-1 11 

effects  of 23 

features  of 111-112 

progress  of 109-111 

rock  resistance  and,  relations  of 113-114 

Eureka,  Nev.,  andesite  from,  analysis  of 219,244 

daciteand  rhyolite  from,  analysis  of 58,65 

F. 

Fault  blocks,  displacement  of,  diagram  showing 4R 

Faulting,  age  of 72 

character  of 22, 141-14(1 

dikes  along 73-74 

finding  of 79-80 

location  of 115-116 

occurrence  of 37-38, 47, 75, 149, 164, 184 

origin  of 21, 47, 72, 80, 146 

principles  of 72-82 

topographic  features  due  to 74-79, 114 

Faulting,  cross,  effects  of 77-79 

theory  of , 157-161 

diagrams  illustrating 158, 159, 160, 161 

Feldspar,  alteration  of 208, 21 1-1!]  •>,  -JIM 

alteration  of,  chemistry  of 230-231 

Fissures,  origin  of 104 

Formations,  geological,  age  of «s-?2 

description  of 30-66 

succession  of 71-72, 274 

Fossils,  occurrence  of 66-67, 69-70 

Fraction  dacite  breccia,  age  of 40 

character  and  occurrence  of 39-40 

section  of,  diagram  showing 40 

Fraction  Extension  shaft,  depth  of 106 

location  of 200 

rocks  in 201-202 

Fraction  faults,  cause  of 146 

description  of 141-146 

discovery  of 79 

effects  of 152 

Wandering  Boy  fault  and,  age  of 163 


INDEX. 


291 


Page. 

Fraction  mine,  faultsin 141-144,147 

faults  in,  figures  showing 141-145,147,162 

gas  in 94 

gold  discovered  in 27, 140 

oxidation  in 91 

plan  of 162 

rock  from,  analysis  of 87 

water  in 108 

shafts  of,  depth  of 106, 140 

earlier  andesite  from,  alteration  of 221-222 

description  of 214 

analysisof 216,221 

rocksof 147-148 

silver  in 94-95 

veinsof 147-148  '. 

plan  showing 162 

Fraction  vein,  character  of 140,146 

discovery  of , 27,140 

relations  of  Valley  View  vein  system  and 140, 187 

Fracture  zones,  development  of,  stages  of 84-85  j 

Friedel  and  Sarasin,  experiments  by 228-229 

Fuel,  expense  of 28 


G.  &  H.  mine,  lava  from,  analysisof 67,60 

rhyolite  from 61 

Galena,  character  of 88 

occurrence  of 22,88 

GaJlionella  granulata.-occurrence  of 70 

marchica,  occurrence  of 70 

procera,  occurrence  of 70 

punctate,  occurrence  of 70 

sculpta.  occurrence  of 70 

tenerrima,  occurrence  of 70 

Gas,  occurrence  of 94, 188 

Geikie,  A.,  on  Antrim  rocks 52-53 

Genth,  F.  A.,  on  albite 212 

jentil,  G.  L.,  on  feldspar  alteration : 212 

Geological  history,  summary  of 66-68, 109, 261-262 

Gold,  depth  and,  relations  of 124-128 

d  iscovery  of 21 

occurrence  of 22, 89, 123, 287 

Gold  Hill,  erosion  on 113 

faulting  at 164 

rocksof 32,164-165 

alteration  of 165 

sections  of 166,167 

veins  of 165-166 

Gold  Hill  fault,  location  of 164,184 

Gold  Hill  mine,  relations  of 186-187 

Gold  Hill  shaft,  depth  of 166 

water  in 105-106 

Golden  Anchor  shaft,  depth  of 205 

rocks  in 206 

water  in 106 

Golden  Mountain,  characterof 44 

erosion  on 113 

lava  of,  analyses  of 57, 58, 60, 66 

cooling  of,  eddying  in,  figure  showing 46 

rocksof 36,45,55,60-fi2 

section  near,  diagram  showing 45 

Good  Enough  shaft,  section  on 166, 167 

Good  Enough  vein,  characterof 165 

output  of 166 

structure  of 166 

Granite,  occurrence  and  character  of 30 


Page 

Great  Basin,  deformation  in 80-81 

erosion  in 111-112 

lavas  of,  succession  of 68-69, 274 

metallographic  province  of 22,276-278 

petrographic  province  of 22, 274-275 

Gypsum,  formation  of 94 

H. 

Halifax  shaft,  later  andesite  from, analysisof.  57,241,244-247 

later  andesite  from,  description  of 239 

study  of 244-248 

location  of 205 

rocks  in 205 

water  in 105,205 

Healy,  J.  M.,  information  from 152 

Heller  Butte,  character  of 87 

erosion  on 113 

rocks  in 30, 37 

view  of 38 

Heller  dacite,  age  of 38 

character  of    37-39 

occurrence  of 30, 37 

Hillebrand,  W.  F.,  analyses  by 87,89-90,92,103,241 

History,  geological,  summary  of 66-68, 109 

Hobbs,  W.  H,  diagram  by 217 

Hornblende,  alteration  of 207 

occurrence  of 63 

I. 

Igneous  rocks,  classification  of 59 

occurrence  of 21 

relative  ages  of 68-72 

Infusoria,  occurrence  of 69-70 

J. 

Japan,  rocksof 280-282 

K. 

Kaolin,  occurrence  of 209 

Kawsoh  Mountain,  fossils  from 70 

King,  Clarence,  on  Miocene  deposits 69-70 

King  Tonopah  shaft,  depth  of 106 

lava  from,  analysis  of 64 

rocks  and  veins  in 197-198 

L. 

Lake,  ancient,  location  and  history  of 51-54, 67, 81 

Lavas,  analyses  of 57 

boundaries  of 74 

characterof 21.31-66,56 

cooling  of,  eddying  in,  diagram  showing 46 

occurrence  of 31-66 

origin  of,  differentiation  theory  of 61-66 

relations  of,  diagrams  showing 40, 41, 43-45, 48, 49, 53 

succession  of 68-69 

water  in 255 

Limestone,  occurrence  and  character  of 30 

Lindgren,  W.,  on  propylite 236 

Little  Tonopah  shaft,  data  concerning 199-200 

Lode  porphyry,  mention  of 31 

See  aim  Andesite,  earlier. 
Lone  Mountain,  racks  on 30 

M. 

Macdonald  vein,  characterof 174-175 

occurrence  of 169 

section  of,  figure  showing 174, 175 


292 


INDEX. 


Page. 

UacNamara  mine,  gas  in 94 

location  of 189 

shaft  of,  depth  of 106 

rocks  in 189-191 

section  on 191 

Magmatic  segregations,  theory  of 21-22 

Map  of  andesite  veins  in  Mizpah  workings 183 

of  Ararat  Mountain 102 

of  Tonopah  mining  claims 26 

of  Tonopah  mining  district 21,114 

of  Tonopah  outcropping  veins 84 

Map,  diagrammatic,  showing  zigzag  scarps 76 

Map,  geologic,  of  Tonopah  mining  district 56, 114 

Melosira  granulata,  occurrence  of 69-70 

punctata,  occurrence  of 70 

varians,  occurrence  of 69 

Mesabi  range,  sideritein 249 

Metallographic  province,  extent  of 22, 276 

ores  of,  origin  of 276-278 

Mexico,  ores  of,  comparison  of  Tonopah  ores  and ...  267-270 

petrographic  province  of 274-275 

Mica,  production  of 232-233 

Midway  mine,  location  of 179 

ores  of,  character  of 86, 181 

rocks  in 35,179-180 

section  in,  figure  showing 180 

shaft  of,  depth  of 106 

veins  of 180-181 

Milling,  deficiencies  of 28 

Mineral  veins.    See  Veins,  mineral. 

Mineralization,  agents  of 85, 253-264 

origin  of 22,258-262 

period  of 83,261 

See  also  Water,  hot,  ascending. 

Mines,  descriptions  of 115-206 

plotof 26 

Miocene  time,  history  in 67-69 

Miriam  claim,  description  of 193 

Mizpah  Extension  shaft,  later  andesite  of,  analysis 

of 241,244 

later  andesite  of,  description  of 238-239 

study  Of 244-245 

lava  of,  analysis  of 57 

location  of 194 

rocks  in 194-195 

temperatures  in 263-264 

diagram  showing 265 

veins  in 195-197 

water  in 106 

Mizpah  fault,  discovery  of : 79 

location  of 115,126-127,168-169,177 

Mizpah  Hill,  alteration  on 207 

earlier  "r^esite  from,  analysis  of 216, 223, 225 

description  of 214-215 

study  of 223-225 

erosion  on 113-114 

faulting  on 74 

gold  found  at 25-26 

outcropping  veins  on,  view  of 116 

rocks  on  and  near 32,110,164 

underground  temperatures  at 264 

veins  of 83 

plan  of 120 

view  from 46 

Mizpah  mine,  earlier  andesite  from,  analysis  of..  216, 225,226 

earlier  andesite  from,  description  ol 214-215 

study  of 225-226 


Page. 

Mizpah  mine,  faults  in,  figure  showing 122,123 

ores  of,  character  of 86, 132 

rocks  of 207 

section  in 173 

silver  in 95 

veins  of,  diagram  showing 122-124, 126, 183 

Mizpah  vein,  alterations  in 124-125 

branching  of 119 

diagram  showing 120 

composition  of 122-123 

earlier  audesite  from,  analysis  of 216,226,226 

description  of 215-216 

study  of 225-221J 

extent  of 115-117 

faults  in 115-117 

plan  showing 123 

fractures  in 119-122 

gold  in 124-125 

junction  of  Burro  vein  and 127 

minerals  of 124 

occurrence  of 126, 173-174 

ores  of,  distribution  of,  diagram  showing 121 

oxidation  of 90, 124-125 

relations  of,  figure  showing 173 

sections  of,  diagram  showing 116, 117, 119-124, 173 

structure  of 117 

Mizpah  vein  system,  description  of 115-129 

Molly  shaft,  data  concerning 106,200 

Monitor  Valley,  Nev.,  hot  springs  in 257 

Mono,  Lake,  crater  at 82, 257 

Montana  Tonopah  mine,  faults  in 172 

faults  in,  plan  showing 168 

galena  in 88 

gold  of 27, 125 

ores  of 86, 95, 175 

analysis  of 89 

oxidation  in 91 

sections  in 169,170,171,174,175,176 

shaft  of,  depth  of 106 

later  andesite  in,  analysis  of 241,247 

description  of 240-241 

study  of 247-248 

temperatures  in 264 

diagram  showing 265 

veins  of 85, 167-173 

figures  showing 169, 170, 171 

formation  of 172 

structure  in 169-170 

Montana  Tonopah  vein  system,  description  of 168-184 

Montana  vein,  character  of 170-172,174 

faults  on 172,174 

occurrence  of 170, 178 

ore  of,  figure  showing 84, 171 

origin  of 172 

relations  of,  figu  re  showing 173 

vein  of,  fragment  of,  view  of 84 

Muscovite,  formation  and  occurrence  of 231-233 


Nelhart,   Mont.,  silver  sulphides   at,  comparison  of 

•     Tonopah  sulphides  and 95 

Nevada,  springs  of 256-259 

New  York  Tonopah  shaft,  depth  of 106,200 

location  of 200 

rocksin 30,39-10,110,200-201 

New  Zealand,  rocks  of 279-280, 284-286 

Nickel,  occurrence  of 34 


INDEX. 


293 


Page. 

North  Star  mine,  discovery  of  gold  in 27 

faultsin 178 

ores  of,  character  of 86 

section  in 177-178 

figure  showing '. 177 

shaft  of,  depth  of 106 

later  andesite  from,  analysis  of 241, 246 

description  of 239-240 

study  of 246-247 

rocksin 126,177-178 

veins  in 178 

O. 

Oddie,  Mount,  erosion  on 113 

lava  from,  analysis  of 58,60 

rocksof :.  36,49-51,53,60-61,101 

view  from 24 

view  of 52 

Oddie,  T.  L.,  mention  of 26 

Oddie  rhyolite,  age  of 50 

character  of 49-50 

contact  of,  section  showing 193 

veins  at 191-194 

lava  from,  analysis  of 57 

occurrence  of 49, 127 

rhyolile  from 61 

Oddie  shaft,  section  in,  diagram  showing 116 

Ohio  Tonopah  shaft,  depth  of 106-203 

location  of •. 202 

minerals  in 205 

roctsof 107,202-205 

section  near,  diagram  showing 43 

temperatures  in 263-204 

diagram  showing 265 

Ordonez,  E.,  on  Mexican  ores 267-268,273 

on  petrographic  province 274 

Ore  production,  amount  of ---  26,28 

Ore  shoots,  origin  of 85,119-122,276-278 

Oregon,  craters  in 82 

Ores,  at  Tonopah  and  elsewhere,  comparison  of.- ..  267-287 

genesis  of 261-262 

treatment  of - 28 

Ores,  oxidized,  analysisof _ 92 

Ores,  primary,  composition  of 86-90 

location  of 86 

Oxidation,  agents  of 90 

depth  of 22,90 

effects  of 90-94 

process  of 93-94 

V. 

Pachuca,  Mexico,  ores  of  Tonopah  and,  comparison  of      22, 

267-268,273,278 

section  at,  figure  showing 277 

Pachuca  Range,  character  of 267 

veins  of 267-268 

Pacific  petrometallographic  zone,  existence  of 278-287 

Pah-Ute  Lake,  deposits  of,  comparison  of  Siebert  tuffs 

and 70 

Paleozoic  limestone,  occurrence  of 66 

Paragenesis  of  vein  material 104 

Pelee,  Mont,  plug  of 104 

Penrose,  R.  A.  F.  jr.,  on  oxidized  veins 91 

Peru,  rocks  of 280 

Petrcgraphic  province,  extent  of 22, 274-275 

Petroleum ,  use  of 29 

Petrometallographic  zone.  Pacific,  existence  of 278-287 


Page. 

Phosphorus,  alteration  of 233-234 

Physiography  of  region,  account  of 109-114 

Pinnubaria  inaequalis,  occurrence  of 70 

Pktsburg  shaft,  data  concerning 204 

Planorbis  utahensis,  occurrence  of 67 

Pliocene  time,  history  in 68, 69, 110 

1'olybasite,  occurrence  of 22,95 

Power,  use  of 28 

Propylite,  definition  of 236-237 

Prospects,  descriptions  of 115-206 

Pseudomorphs,  character  of 61-62 

origin  of 62 

Pyrargyrite,  occurrence  of 22, 94-95 

Pyrite,  character  of 88 

occurrence  of 22 

relations  of  siderite  and 208 

view  of  rock  specimen  containing 208 

Q. 

Quartz,  alteration  of  earlier  andesite  to 207-209 

analysis  of 87 

character  of 86 

occurrence  of 22 

origin  of 22 

Quaternary  erosion,  sketch  of 109-111 

Quinn  Canyon  Range,  view  of 112 

R. 

Railroad,  construction  of 25, 28 

Rainfall,  absorption  of 107-108 

amount  of 112-113 

Ray,  fossils  near 66-67 

Real  del  Monte,  ores  of  Tonopah  and,  comparison  of.    267- 

268,273-274 

Red  Rock  shaft,  data  concerning 204 

Replacement,  veins  due  to 84-S5 

Reptile  elaim,  vein  on 103 

vein  on,  section  of,  diagram  showing 102, 103 

Rescue  shaft,  description  of 194 

water  in 105 

Rhynlitc,  analyses  of 57-58,60,64,65 

areas  of,  diagram  showing 46 

character  of 21,36-51, S9-SS 

classification  of 59-iiO 

fissure  veins  in 102 

flow  brecciation  in 102 

hornblende  in 62 

occurrence  of 21, 36-51 

origin  of 22,63 

veins  in 22 

Richthofen,  F.  von,  on  mineralization 259-260 

on  propylite 236 

Rock  alteration,  changes  during,  diagrams  illustrat- 
ing    218,234,242,243 

maximum  points  of,  location  of 226-227 

of  the  earlier  andesite 207-238 

of  the  later  andesite 238-252 

of  the  Oddie  rhyolite 252 

processes  of 207-252 

Rocks,  altered,  specimens  of,  analyses  of 216 

microscopic  description  of 213-216 

phases  of,  diagrams  showing 217-218 

study  of 217-226 

Rosenbusch,  H.,  on  feldspar 233 

on  propylite 236 

Rushton  Hill,  rocksof  and  near 43,49-50,57,60-61 


294 


INDEX. 


8.  Page. 

San  Antonio  Range,  location  and  origin  of 109 

Sarasin  and  Friedel,  experiments  by 228-229 

Scarps,  rock,  origin  of 74-79,113-114 

view  of 76 

Schaller,  W.  T.,  analysis  by 88,178 

Scrope,  G.  P.,  on  volcanic  subsidence 47 

Sedimentary  rocks,  occurrence  of 21 

Selenium,  occurrence  of '. 92, 281, 285 

Sericite,  adularia  and,  relations  of 227-228 

alteration  of  earlier  andesite  to 207-209 

character  of 87 

occurrence  of 22 

origin  of 22 

Shafts,  depth  of 106-107 

Siderite,  formation  of 248-249 

relations  of  pyrite  and 208 

view  of 208 

Siebert  fault,  location  of 115-117 

Siebert  Mountain,  character  of 44 

erosion  on 113 

events  on 67-68 

faulting  near 75 

fossils  at 69-70 

lava  from,  analysis  of 57, 64 

rocks  of 36,40,45,53-55 

section  near,  diagram  showing 53 

veins  near 97, 99 

view  of 52, 54 

Siebert  shaft,  depth  of 106 

earlier  andesite  from,  analysisof , 216 

description  of 213 

study  of 217,219-220 

rocks  in 116-117 

section  on,  figure  showing 134 

view  of 118 

Siebert  tuffs,  boundaries  of 73 

character  of 51 

comparison  of  Pah-Ute  Lake  deposits  and 70 

erosion  of 114 

fossils  in 69-70 

occurrence  of 21, 47-48 

origin  of 51-54 

relations  of,  diagram  showing 53 

section  of,  diagram  showing 45 

view  of 46 

Silurian  rocks,  occurrence  of 30 

Silver,  discovery  of 21 

occurrence  of 91, 94-95, 123-124, 146, 164, 269, 287 

Silver,  horn.    Hee  Silver  chlorides. 

Silver  chlorides,  character  of 88, 91 

occurrence  of 122, 180-181 

Silver  City,  Idaho,  ores  of  Tonopah  and,  comparison 

of 22,271-274 

Silver  Peak,  crater  at 82,257 

crater  at,  view  of 112 

hot  and  cold  springs  at 256-257 

Silver  Peak  Range,  coal  in 29 

rocks  in 30 

Hilver  selenides,  occurrence  of  . . .'. 22,92 

Silver  sulphides,  occurrence  of 22, 88, 94-%,  180-183 

Silver  Top  sha ft,  rocks  in 125 

section  in,  figure  showing 136 

veins  in 136-137 

relation  of  Stone  Cabin  vein  and 137-139 

relation  of  Valley  View  vein  and 139 

water  in 106 

Smelting,  necessity  for 28 

Sodavllle,  Nev..  rainfall  at 112-113 


Page. 

Solfataras,  action  of,  nature  of 260-261 

Spha-riuin  idahoense,  occurrence  of 67 

Spongolithis  acicularis,  occurrence  of 70 

Springs,  hot,  extinction  of 258 

origin  of 23,254-2511 

See  also  Water,  hot  ascending. 

Steiger,  George,  analyses  by 148, 216, 241 

Stephanite,  occurrence  of : 22 

Stock,  A.  C.,  aid  from 198-199 

Stone  Cabin  fault,  extent  of 139 

Stone  Cabin  mine,  ore  of,  character  of 136 

oxidation  in 91 

sections  of 135, 138 

shaft  of,  depth  of lOti 

veins  in 135-136 

relation  of  Silver  Top  vein  and 137 

relation  of  Valley  View  vein  and 137-139 

figure  showing 138 

Suess,  E.,  on  underground  water 255, 258 

on  volcanoes 261 

Sulphide  ores,  primary,  analysis  of 89-90 

Sumatra,  rocks  of 283-284 

Summary  of  paper 21-23 

T. 

Tellurium,  occurrence  of iix;» 

Temperatures  at  Comstock  and  Tonopah,  comparison 

Of 265-266 

in  the  Mizpah  Extension 263-264 

in  the  Mizpah  Hill  mine 264 

in  the  Montana  Tonopah 264 

in  the  Ohio  Tonopah 263-264 

measurements  of,  method  of 263 

on  the  Comstock 26n 

relations  of  depth  and 23, 263-26t; 

diagrams  showing 265 

Tertiary  rocks,  character  of 31-66 

occurrence  of 21, 31-66, 68 

Tertiary  time,  history  in 66, 109-110 

Titanium ,  alteration  of 233-234 

Tonopah,  character  of 27 

cross  section  at,  figure  showing ; 71 

hot  springs  near 257-25S 

lavas  of,  succession  of 68-«9, 274 

location  of 25 

name  of,  meaning  of 27 

outcrops  at,  diagram  showing  84 

rocks  of,  age  of 69-72 

view  of 27, 46 

Tonopah  and  California  mine,  description  of 167-168 

fault  in 167 

ores  of,  character  of 86 

rocks  in 53 

section  in 167 

shaft  of,  depth  of 106 

earlier  andesite  from,  analysis  of 216,220 

description  of 213 

study  of 220-221 

section  near,  diagram  showing 40 

veins  in His 

Tonopah  City  shaft,  depth  of 106,202 

location  of 202 

rocks  In 37-38,202 

Tonopah  Extension  mine,  discovery  of  gold  in 27 

rocks  in 35,181-184 

sections  in,  figures  showing 182, 191 

shaft  of,  depth  of 106 

vcinsin 182-184 


INDEX. 


295 


Page. 

Tonopah  Mining  Co.,  claims  of,  development  of '26-27 

shaft  of,  view  of 120 

Tonopah  rhyolite-dacite,  age  of 43, 51 

alteration  in 41-42 

analysis  of 57,58,60,64,148 

character  of 41-13, 51, 59-«l,  101 

contact  of,  veins  on 194-200 

hornblende  in £2 

occurrence  of 30,41,51,127 

section  of,  figure  showing 41 

veins  of 22,96-101 

age  of 99-100 

character  of 97-99 

circulating  waters  in 100-101 

diagram  showing 98 

limits  of 99-100 

Topography,  character  of 25 

origin  of 23,109,113-114 

production  of,  by  faults 74-75 

relation  of  rocks  to 113-114 

Transportation,  difflcultieH  of 28 


V. 


Valley  View  fault,  effects  of 137-139 

location  of 133-134 

Valley  View  mine,  shaft  of,  depth  of 106 

shaft  of,  sections  on  and  near,  figures  showing . .  122, 134 

workings  of 132-134 

Valley  View  veins,  correlation  of 137-139 

cross  veins  in 129-130 

location  of 129 

onMizpah  Hill 129-132 

ores  of,  character  of 132 

origin  of 130-131 

oxidation  in 90-91 

oxidized  ore  of.  analysis  of 92 

sections  of 128,133-138 

structure  in 130-131 

underground  system  of ; 132-139 

in  Silver  Top  mine 136-187 

in  Stone  Cabin  mine 135-137 

Valley  View  vein  system,  description  of 129-1C4 

relations  of  Fraction  vein  and 140, 1S7 

relations  of  Wandering  Boy  vein  and 149-152, 187 

section  of,  figure  showing 151 

Van  Hise,  C.  R..  on  orthoclase 229 

Vein  robbers,  occurrence  and  character  of 130 

Veins,  mineral,  age  of 71-72 

branching  of 119 

character  of ..  22, 83-101, 122-123 

formation  of 104 

relations  of  alteration  to 251-252 

fractures  in 119-122 

in  the  earlier  andesite 83-90 

mines  and  prospects  on,  descriptions  of 115-191 

material  of,  paragenesis  of 104 

of  Ararat  Mountain : 101-104 

of  the  Tonopah  rhyolite-dacite  period 96-101 

origin  of 84-85,102,104,253-262 

outcrops  of,  map  showing 116 

Vivipara  couesi,  occurrence  of 67 

Volcanic  epoch,  continuance  of S2 


Page. 

Volcanoes,  accumulations  from 23 

character  of 260-261 

origin  of 21 

W. 

Wandering  Boy  fault,  discovery  of 79 

Fraction  fault  and,  age  of 163 

location  of 153 

occurrence  and  character  of 152-156, 161-163, 184 

section  showing 154, 155, 156 

Wandering  Boy  mine,  faults  in 152-156, 161-162 

faults  in ,  figure  showing 172 

ore  in 163 

outcrops  near,  figure  showing 153 

oxidation  in 91 

plan  of •  172 

relations  of 186 

section  of 154, 155. 156 

shafts  of.  depth  of 106 

Valley  View  veins  in 130, 149-152 

veins  of : 1W2-163 

plan  showing 172 

Wandering  Boy  veins,  description  of 149-164 

dip  of H9-151 

Fraction  vein  and,  relations  of 162-163, 187 

ores  of 163-164 

outcrops  of 152 

Valley  View  vein  and,  relations  of 149-152 

Wash  apron,  view  of 112 

Washoe,  Nev.,  andesite  from,  analyses  of 219, 244 

dacite  and  rhyolite  from 60 

analyses  of • 58, 65 

Water,  supply  of 28, 107 

Water,  underground,  depth  to 106-107 

occurrence  of 23, 105-107 

origin  of 107-108 

zones  of, 'occurrence  of.  explanation  of 107 

Water,  descending,  oxidation  by 90 

sulphides  deposited  by % 

Water,  hot,  ascending,  alteration  by 207-252 

alteration  by,  variation  in 210-211 

changes  in 235-238 

channels  of 83 

effect  of 22,114,227.234-235 

mineral  composition  of 22- 

23, 104. 210-211. 227. 235-238, 250, 253, 2SN-25H 

argument  from 258-260 

mineralization  by 22-23,*% 99-101, 210-211 

origin  of 253-256 

Water  power,  availability  of 28 

Weed,  W.  H.,  on  Boulder  Hot  Springs 211 

on  silver  sulphides 95 

West  End  fault,  location  of 184 

rocks  along .' 184 

West  End  mine,  gas  in 94 

relations  of 186-187 

rocks  in 35.50;  185-188 

shaft  of,  depth  of 106,185 

workings  of,  description  of 184-188 

White  Mountain  Range,  water  power  in 28 

Wind,  erosion  by 110-112 

Wingfield  tunnel,  location  of 191 

rocks  in 101,104,191-192 

Wood,  occurrence  of 28 


o 


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PP  15.  The  mineral  resources  of  the  Mount  \Vrangell  district,  Alaska,  by  \V.  C.  Mendenhall  and  F.  C.  Schrader.    1903. 

71  pp.,  10  pis. 

B  218.  Coal  resources  of  the  Yukon,  Alaska,  by  Arthur  J.  Collier.    1903.    71  pp..  (i  pis. 
B  219.  The  ore  deposits  of  Tonopah,  Nevada  (preliminary  report),  by  J.  E.  Spurr.    1903.    31  pp.,  1  pi. 
PP  20.  A  reconnaissance  in  northtrn  Alaska  in  1901,  by  F.  C.  Schrader.    1904.    139  pp.,  16  pis. 
PP  21.  Geology  and  ore  deposits  of  the  Bisbee  quadrangle,  Arizona,  by  F.  L.  Ransome.    1904.    168  pp.,  29  pis. 
B223.   Gypsum  deposits  ill  the  I'nited  States,  by  G.  I.  Adams  and  others.    1904.    129  pp.,  21  pis. 
I'P  24.  Zinc  and  lead  deposits  of  northern  Arkansas,  by  G.  I.  Adams,  assisted  by  A.  H.  Purdue  and  E.  F.  Burchard,  with  a 

section  on  the  determination  and  correlation  of  formations,  by  E.  O.  Ulrich.    1904.    118  pp.,  27  pis. 
PP  25.  The  copper  deposits  of  the  Encampment  district,  Wyoming,  by  A.  C.  Spencer.    1904.    107  pp.,  2  pis. 
B  225.  Contributions  to  economic  geology,  1903.  by  S.  F.  Emmons  and  C.  W.  Hayes,  geologists  in  charge.    1904.    527  pp.  1  pi. 
PP  26.  Economic  resources  of  the  northern  Black  Hills,  by  J.  D.  Irving,  with  contributions  by  S.  F.  Emmons  and  T.  A. 

Jaggar,  jr.    1904.    222  pp.,  20  pis. 
PP27.  A  geological  reconnaissance  across  the  Bitterroot  Range  and  Clearwater  Mountains  in  Montana  and  Idaho,  by 

Waldenmr  Liiulgren.     1904.    122  pp. .  15  pis. 

B  229.  Tin  deposits  of  the  York  region,  Alaska,  by  A.  J.  Collier.  1904.  61  pp.,  7  pis. 
B  236.  The  Porcupine  placer  district,  Alaska,  by  C.  W.  Wright.  1904.  35  pp.,  10  pis. 
B  238.  Economic  geology  of  the  lola  quadrangle.  Kansas,  by  (;.  I.  Adams,  Erasmus  Haworth,  and  W.  R.  Crane.  1904.  83 

pp.,  11  pis. 

B  243.  Cement  materials  and  industry  of  the  United  States,  by  E.  C.  Kckel.    1905.    395  pp.,  15  pis. 
B  246.  Zinc  and  lead  deposits  of  northwestern  Illinois,  by  H.  Foster  Bain.    1MM.    M  pp.,  5  pis. 
B  247.  The  Fairhavcn  gold  placers,  Seward  Peninsula.  Alaska,  by  F.  II.  Mottit.    190ft.    85  pp..  14  pis. 
B249.  Limestones  of  southeastern  Pennsylvania,  by  F.  G.  Clapp.    1905.    52  pp..  7  pis. 
B  250.  The  petroleum  fields  of  the  Pacific  coast  of  Alaska,  with  an  account  of  the  Bering  River  coal  deposits,  by  G.  C. 

Martin.    1905.    64  pp..  7  pis. 

B251.  The  gold  placers  of  the  Fortymile,  Birch  Creek,  and  Fairbanks  regions,  Alaska,  by  L.  M.  Prindle.    1905.    89  pp.,  16  pis. 
\\S  117.  The  lignite  of  North  Dakota  and  its  relation  to  irrigation,  by  F.  A.  Wilder.    1905.    59  pp.,  8  pis. 
PP  36.  The  lead,  zinc,  and  fluorspar  deposits  of  western  Kentucky,  by  E.  ().  Ulrich  and  W.  S.  T.  Smith.    1905.    218  pp.,  15pls. 
PP  38.  Economic  geology  of  the  Bingham  mining  district  of  Utah,  by  J.  M.  Boutwell,  with  a  chapter  on  areal  geology,  by 

Arthur  Keith,  and  an  introduction  on  general  geology,  by  S.  F.  Emmons.    1905.    413  pp.,  49  pis. 
PP  41.  The  geology  of  the  central  Copper  River  region,  Alaska,  by  W.  C.  Mendenhall.    1906. 
B  254.  Report  of  progress  in  the  geological  resurvey  of  the  Cripple  Creek  district,  Colorado,  by  Waldemar  Lindgren  and 

F.  L.  Kiinsome.    1904.    36  pp. 

B  255.  The  fluorspar  deposits  of  southern  Illinois,  by  H.  Foster  Bain.    1905.    75  pp.,  6  pis. 
B  256.  Mineral  resources  of  the  Elders  Ridge  quadrangle,  Pennsylvania,  by  R.  W.  Stone.    1905.    86  pp.,  12  pis. 
B  259.  Report  on  progress  of  investigations  of  mineral  resources  of  Alaska  in  1904,  by  A.  H.  Brooks  and  others.    1905. 

196  pp.,  3  pis. 

B  260.  Contributions  to  economic  geology,  1904;  S.  F.  Emmons,  C.  W.  Hayes,  geologists  in  charge.    1905.    620  pp.,  4  pis. 
B  261.  Preliminary  report  on  the  operations  of  the  coal-testing  plant  of  the  United  states  Geological  Survey  at  the  Louisiana 

Purchase  Exposition,  St.  Louis,  Mo.,  1904;  E.  W.  Parker,  J.  A.  Holmes.  M.  R.  Campbell,  committee  in  charge.    1905. 

172  pp. 

B  26S.  Methods  and  costs  of  gravel  and  placer  mining  in  Alaska,  by  C.  W.  Purington.    1905.    27:1  pp.,  42  pis. 
PP  42.  Geology  of  the  Tonopah  mining  district.  Nevada,  by  J.  E.  Spurr.    1906.    295  pp.,  -Jl  |.K 

SERIES  B.   DESCRIPTIVE  GEOLOGY. 

B23.  Observations  on  the  junction  between  the  Eastern  sandstone  and  the  Kcwceiiaw  series  on  Kcweenaw  Point,  Lake 

Superior,  by  K.  D.  Irving  and  T.  C.  Chamberlin.    1885.    124  pp.,  17  pis.     (Out  of  stock.) 
B  33.  Notes  on  geology  of  northern  California,  by  J.S.  Dillcr.    1886.    23pp.    (Out  of  stork.) 

B  39.  The  upper  beaches  and  deltas  of  Glacial  Lake  Agassiz,  by  Warren  Upham.    1887.    84  pp.,  1  pi.    (Out  of  stock.) 
B  40.  Changes  in  river  courses  in  Washington  Territory  due  to  glaciation,  by  Hailey  Willis.    1887.    10  pp.,  4  pis.    (Out  of 

Ktock.) 

B  45.  The  present  condition  of  knowledge  of  the  geology  of  Texas,  by  R.  T.  Hill.    1887.    94  pp.    (Out  of  stock.) 
B  53.  The  geology  of  Nantucket,  by  N.  8.  Shaler.    1889.    55  pp.,  10  pis.    (Out  of  stock.) 
B  57.  A  geological  reconnaissance  in  southwestern  Kansas,  by  Robert  Hay.    1890.    49  pp.,  2  pis. 

K  >.  The  glacial  txmndary  in  western  Pennsylvania,  Ohio,  Kentucky.  Indiana,  and  Illinois,  by  G.  F.  Wright,  with  intro- 
duction by  T.  C.  Chamberlln.    1890.    112pp.,  8  pis.    (Out  of  stock.) 


ADVERTISEMENT.  Ill 

B  67.  The  relations  of  the  traps  of  the  Newark  system  in  the  New  Jersey  region,  by  N.  H.  Darton.    1890.    82  pp.    (Out  of 

stock.) 

B  104.  Glaciation  of  the  Yellowstone  Valley  north  of  the  Park,  by  W.  H.  Weed.    1893.    41  pp.,  4  pis. 
BIOS.  A  geological  reconnaissance  in  central  Washington,  by  I.  C.  Russell.    1893.    108  pp.,  12  pis.    (Out  of  stock.) 
B  119.  A  geological  reconnaissance  in  northwest  Wyoming,  by  G.  H.  ElcJridge.    1894.    72  pp.,  4  pla. 
B  137.  The  geology  of  the  Fort  Riley  Military  Reservation  and  vicinity,  Kansas,  by  Robert  Hay.    1896.    35  pp.,  8  pis. 
B  144.  Tlu-  moraines  of  the  Missouri  Coteau  and  their  attendant  deposits,  by  J.  E.  Todd.    18%.    71  pp.,  21  pis. 
B  158.  The  moraines  of  southeastern  South  Dakota  and  their  attendant  deposits,  by  J.  E.  Todd.    1899.    171  pp.,  27  pis. 
B  159.  The  geology  of  eastern  Berkshire  County,  Massachusetts,  by  B.  K.  Emerson.    1899.    139  pp.,  9  pis. 
B  165.  Contributions  to  the  geology  of  Maine,  by  H.  S.  Williams  and  H.  E.  Gregory.    1900.    212  pp..  14  pis. 
WS  70.  Geology  and  water  resources  of   the  Patrick  and  Goshen  Hole  quadrangle!)  in  eastern  Wyoming  and  western 

Nebraska,  by  G.  I.  Adams.    1902.    50  pp.,  11  pis. 

B  199.  Geology  and  water  ^resources  of  the  Snake  River  Plains  of  Idaho,  by  I.  C.  Russell.    1902.    192  pp.,  25  pis. 
PP  1.  Preliminary  report  on  the  Ketchikan  mining  district.  Alaska,  with  an  introductory  sketch  of  the  geology  of  south- 

.•intiTii  Alaska,  by  A.  H.  Brooks.    1902.    120  pp.,  2  pis. 

PP  2.  Reconnaissance  of  the  northwestern  portion  of  Steward  Peninsula,  Alaska,  by  A.  J.  Collier.  1902.  70  pp.,  11  pis. 
PP  3.  Geology  and  petrography  of  Crater  Lake  National  Park,  by  J.  S.  Diller  and  H.  B.  Patton.  1902.  167  pp.,  19  pis. 
PP  10.  Reconnaissance  from  Fort  Hamlin  to  Kotzebuc  Sound.  Alaska,  by  way  of  Dall,  Kanuti.  Allen,  and  Ko\yak  rivers, 

by  W.  C.  Mendenhall.    1902.    68  pp.,  10  pis. 

PP  11.  Clays  of  the  United  States  east  of  the  Mississippi  River,  by  Heinrich  Ries.    1903.    298  pp.,  9  pis. 
PP  12.  Geology  of  the  Globe  copper  district,  Arizona,  by  F.  L.  Ransome.    1903.    168  pp.,  27  pis. 
I'P  13.  Drainage  modifications  in  southeastern  Ohio  and  adjacent  parts  of  West  Virginia  and  Kenutcky,  by  W.  G.  Tight. 

1903.    Ill  pp.,  17  pis. 
B  -IK  Descriptive  geology  of  Nevada  south  of  the  fortieth  parallel  and  adjacent  portions  of  California,  by  J.  E.  Spurr. 

1903.  229  pp.,  8  pis. 

B  209.  Geology  of  Ascutney  Mountain,  Vermont,  by  R.  A.  Daly.    1903.    122  pp.,  7  pis. 

ws  TV  Preliminary  report  on  artesian  basins  in  southwestern  Idaho  and  southeastern  Oregon,  by  I.  C.  Russell.    1903. 

51  pp.,  '_'  pl>. 
PP  15.  Mineral  resources  of  the  Mount  Wrangell  district.  Alaska,  by  W.  C.  Mendenhall  and  F.  C.  Schrader.    1903.    71  pp., 

10  pis. 
1'P  17.  Preliminary  report  on  the  geology  and  water  resources  of  Nebraska  west  <if  the  one  hundred  and  third  meridian. 

by  N.  H.  Darton.     1903.     69  pp.,  43  pis.  , 

B  217.  Notes  on  the  geology  of  southwestern  Idaho  and  southeastern  Oregon,  by  I.  C.  Russell.    1903.    83  pp.,  18  pis. 
B  219.  The  ore  deposits  of  Tonopah,  Nevada  (preliminary  report),  by  J.  E.  Spurr.    1903.    31  pp.,  1  pi. 
PP  20.  A  reconnaissance  in  northern  Alaska  in  1901,  by  F.  C.  Schrader.    1904.    139  pp.,  16  pis. 

PP21.  The  geology  and  ore  deposits  of  the  Bisbee  quadrangle,  Arizona,  by  F.  L.  Ransome.    1904.    168  pp.,  29  pis. 
WS  90.  Geology  and  water  resources  of  part  of  the  lower  James  River  Valley,  South  Dakota,  by  J.  E.  Todd  and  C.  M.  Hall. 

1904.  47  pp.,  23  pis. 

PP  25.  The  copper  deposits  of  the  Kncampinent  district,  Wyoming,  by  A.  C.  Spencer.    1904.    107  pp..  2  pis. 

PP  26.  Economic  resources  of  the  northern  Black  Hills,  by  J.  D.  Irving,  with  contributions  by  S.  F.  Emmons  and  T.  A. 

Jaggar,  jr.    1904.    222  pp.,  20  pis. 
PP  27.  A  geological  reconnaissance  across  the  Bitterroot  Range  and  Clearwater  Mountains  in  Montana  and  Idaho,  by 

Waldemar  Lindgren.    1904.    122  pp.,  15  pis. 
PP  31.  Preliminary  report  on  the  geology  of  the  Arbuckle  and  Wichita  mountains  in  Indian  Territory  and  Oklahoma. 

by  J.  A.  TarT,  with  an  appendix  on  reported  ore  deposits  in  the  Wichita  Mountains,  by  H.  F.  Bain.     1904.    97  pp., 

K  pis. 
B  235.  A  geological  reconnaissance  across  the  Cascade  Range  near  the  forty-ninth  parallel,  by  (.;.  O.  Smith  and  F.  C. 

Calkins.    1904.    103  pp..  4  pis. 

B  236.  The  Porcupine  placer  district,  Alaska,  by  C.  W.  Wright.    1904.    35  pp.,  10  pis. 
B  237.  Igneous  rocks  of  the  Highwood  Mountains,  Montana,  by  L.  V.  Pirsson.    1904.    208  pp.,  7  pis. 
B2»H.  Economic  geology  of  the  lola  quadrangle,  Kansas,  by  G.  I.  Adams,  Erasmus  Haworth,  and  W.  R.  Crane.    1904. 

83  pp.,  1  pi. 

PP  32.  Geology  and  underground  water  resources  of  the  central  Great  Plains,  by  N.  H.  Darton.    1905.    433  pp.,  72  pis. 
WS  110.  Contributions  to  hydrology  of  eastern  United  States,  1904;  M.  G.  Fuller,  geologist  in  charge.    1905.    211  pp.,  5  pis. 
B  242.  Geology  of  the  Hudson  Valley  between  the  Hoosic  and  the  Kinderhook.  by  T.  Nelson  Dale.    1904.    63  pp.,  3  pis. 
PP  H4.  The  Delavan  lobe  of  the  Lake  Michigan  Glacier  of  the  Wisconsin  stage  ol  glaeiation  and  associated  phenomena,  by 

W.  C.  Alden.    1904.    106  pp.,  15  pis. 

PP  35.  Geology  of  the  Perry  Basin  in  southeastern  Maine,  by  G.  O.  Smith  arid  David  White.    1905.    107  pp.,  6  pis. 
B  243.  Cement  materials  and  industry  of  the  United  States,  by  E.  C.  Eckel.    1905.    395  pp.,  15  pis. 
B  24(i.  Zinc  and  lead  deposits  of  northeastern  Illinois,  by  H.  F.  Bain.    1904.    66  pp.,  5  pis. 
B  247.  The  Fairhaven  gold  placers  of  Seward  Peninsula,  Alaska,  by  F.  H.  Moffit.    1905.    85  pp.,  14  pis. 
B  249.  Limestones  of  southwestern  Pennsylvania,  by  F.  G.  Clapp.    1905.    52  pp.,  7  pis. 
B  250.  The  petroleum  fields  of  the  Pacific  coast  of  Alaska,  with  an  account  of  the  Bering  River  coal  deposit,  by  G.  C. 

Martin.    1906.    65  pp.,  7  pis. 
B  251.  The  gold  placers  of  the  Fortymile,  Birch  Creek,  and  Fairbanks  regions,  Alaska,  by  L.  M.  Prindle.    1905.    89  pp., 

16  pis. 

WS.  118.  Geology  and  water  resources  of  a  portion  of  cast  central  Washington,  by  F.  C.  Calkins.    1905.    %  pp.,  4  pis. 
B  252.  Preliminary  report  on  the  geology  and  water  resources  of  central  Oregon,  by  I.  C.  Kusaell.    1905.    138  pp.,  24  pis. 


IV  ADVERTISEMENT. 

PP  36.  The  lead,  zinc,  and  fluorspar  deposits  of  western  Kentucky,  by  E.  O.  TJlrich  and  W.  8.  Tangier  Smith.    1905. 

218  pp.,  15  pis. 
fP  38.  Economic  geology  of  the  Binghrtm  mining  district  of  Utah,  by  J.  M.  Boutwell,  with  a  chapter  on  area!  geology,  by 

Arthur  Keith,  and  an  introduction  on  general  geology,  by  S.  F.  Emmons.    1906.    413  pp.,  49  pis. 
PP  41.  The  geology  of  the  central  Copper  River  region,  Alaska,  by  W.  C.  Mendenhall.    1905. 
B  254.  Report  of  progress  in  the  geological  resurvey  of  the  Cripple  Creek  district,  Colorado,  by  Wald«nar  Lindgren  and 

F.  L.  Ransome.    1904.    36  pp. 

B  255.  The  fluorspar  deposits  of  southern  Illinois,  by  H.  Foster  Bain.    1905.    75  pp.,  6  pis. 
B  256.  Mineral  resources  of  the  Elders  Ridge  quadrangle,  Pennsylvania,  by  R.  W.  Stone.    1905.    85  pp.,  12  pis. 
B  257.  Geology  and  paleontology  of  the  Judith  River  beds,  by  T.  W.  Stanton  and  J.  B.  Hatcher,  with  a  chapter  on  the 

fossil  plants,  by  F.  H.  Knowlton.    1905.    174  pp.,  19  pis. 
PP  42.  Geology  of  the  Tonopah  mining  district,  Nevada,  by  J.  E.  Spurr.    1905.    295  pp.,  24  pis. 

Correspondence  should  be  addressed  to — 

THE  DIRECTOR, 

UNITED  STATES  GEOLOGICAL  SURVEY, 

WASHINGTON,  D.  C. 
SEPTEMBER,  1905. 


LIBRARY  CATALOGUE  SLIPS. 

[Mount  each  slip  upon  a  separate  card,  placing  the  subject  at  the  top  of  the 
second  slip.  The  name  of  the  series  should  not  be  repeated  on  the  series 
card,  but  additional  numbers  should  be  added,  as  received,  to  the  first 
entry.] 


Spurr,  Josiah  Edward,  1870- 

.  .  .  Geology  of  the  Tonopah  mining  district, 
g  Nevada,  by  Josiah  Edward  Spurr.  Washington,  Gov't 
•  print,  off.,  1905. 

295,  v  p.    illus.,  XXIV  pi.  (incl.  front.,  maps)  diagrs.    29*  x  23cm.     (U.  S. 
Geological  survey.     Professional  paper  no.  42) 

Subject  series:  A,  Economic  geology,  56;  B,  Descriptive  geology,  65. 

1.  Geology — Nevada. 


Spurr,  Josiah  Edward,  1870- 

.  .  .  Geology  of  the  Tonopah  mining  district, 
1  Nevada,  by  Josiah  Edward  Spurr.  Washington,  Gov't 
I  print,  off.,  1905. 

295,  v  p.     illus.,  XXIV  pi.  (incl.  front.,  maps)  diagrs.    29J  x  23cm.    (U.S. 
Geological  survey.     Professional  paper  no.  42) 

Subject  series:  A,  Economic  geology,  56;  B,  Descriptive  geology,  65. 

1.  Geology — Nevada. 


U.  S.     Geological  survey. 

j  Professional  papers. 

I     no.  42.  Spurr,  J.  E.     Geology  of  the  Tonopah  mining 
district,  Nevada.      1905. 


U.  S.     Dept.  of  the  Interior. 

see  also 
U.  S.     Geological  survey. 


